OmniDocHow to compare the layout and drawings of two PDFs side by side or superimposed
If you want to see the differences in layout, drawings or scanned pages between two PDFs, text comparison often cannot help. This article discusses three visual comparison methods: side-by-side, simultaneous scrolling, and overlay, as well as the steps and check points for comparing two PDFs in the OmniDoc desktop client.
Key points of this article
- The layout, drawings and scanned pages depend on visual differences. Pure text comparison will miss changes in position, lines and annotations.
- Before comparing, confirm whether text can be selected in both documents and whether the page size and number of pages are consistent.
- Side-by-side is suitable for reading page by page, and superposition is suitable for finding the location of changes. The two are better used together.
- Scanned documents can be compared with text after OCR, but the recognition results must be checked paragraph by paragraph and cannot be directly regarded as the final version.
- When the synchronous scrolling is out of sync, it is probably because the number of pages or page heights are different, and you can manually align the page numbers.
After changing the first version of a contract, design drawing or paper layout, you need to confirm what has been changed between the old and new PDFs. If you only look at the text, the table border has moved, a label line on the drawing has changed its direction, and the header logo has become smaller. These changes may be missed. This kind of content should be compared by "seeing" rather than "reading".
This article is written for those who need to compare layouts, drawings or scanned pages, including engineers checking drawing revisions, designers checking typesetting, editors comparing two versions of manuscripts, and administrators checking stamped scans. After reading, you can decide whether to use side-by-side, synchronous scrolling or overlay, know how to compare two PDFs in the OmniDoc desktop client, and use a common method to complete the comparison when there are no special tools.
How to compare the layout and drawings of two PDFs? One sentence conclusion
Let’s talk about the conclusion first: Use side-by-side or synchronous scrolling to read through page by page, and use overlay to find the specific changes. The combination of the two is safe. When the pure text content is the same but the format is different, it can only be discovered through visual comparison.
| your situation | Recommended practices | reason |
|---|---|---|
| I want to check whether the layout of the two versions is consistent from beginning to end. | Side-by-side display, synchronous scrolling to view page by page | The positions of both sides correspond, and the eyes can quickly compare the overall structure of the page. |
| After revising the drawing, I want to find the changed lines and annotations. | Overlay contrast, highlight or discolor the difference | The same parts will overlap, leaving only the differences, and the changes will be clear at a glance |
| Both documents are scanned and I want to compare the text content. | OCR first, then text comparison, while retaining visual comparison | The scans themselves are pictures and cannot be compared word for word without recognition. |
| The two documents have different page numbers, or a new page has been inserted in the middle | Align page numbers first, then compare paragraph by paragraph | Misaligned page order will cause each page to appear "different", interfering with judgment. |
| You only need to confirm the signature or stamp position on a certain page | View this page directly side by side and zoom in | Partial content does not need to be superimposed, which is more efficient. |
Before you start: Confirm these points first
First confirm whether the file is a vector or a scan. Open the PDF and try to select a piece of text with the mouse. The ones that can be selected are usually text versions; the ones that cannot be selected and the whole page looks like a photo are mostly scanned copies. The two types of document comparison have different ideas: the text version can directly compare the text, while the scanned document must first identify the text, or only make a visual comparison.
Check the page size and proportions of both documents. When exporting drawings, the A3 and A4 pages may only be scaled differently, and the proportions may not match when they are placed side by side. After superposition, the illusion of "overall misalignment" will appear. Check the page size first, and re-export it at the same size if necessary, so that the comparison result will be reliable.
Clear the purpose of comparison. If you just want to see whether the layout is consistent, focus on page structure, fonts and spacing; if you want to find out specific changes, you need to look at where the content has changed. The purpose is different, the selection method and the focus are also different.
Confirm whether the page number and page order correspond. If an extra page is inserted in the new version, or a page is missing, all subsequent pages will not match. Align with page numbers or chapter titles first, then start comparing page by page.
Be prepared for the time; don't change it while watching. Overlay contrasts and page-by-page reading both require focus. Placing two files in the same window, reading through them to note any doubts, then cross-checking them one by one is much more efficient than switching back and forth.
Steps to compare two PDFs in the OmniDoc desktop client
The PDF editor in the OmniDoc desktop client includes a "compare two PDFs" feature. The following steps use it as an example; you can follow the same approach in other environments.
- Open download.omnidoc.top to download and install the desktop client, which supports Windows, macOS, and Linux.
- Launch the client, enter the PDF editor, and open the first PDF, such as an older version of the drawing or draft.
- Find the comparison function and import a second PDF, such as a new version of the file.
- View the comparison results and check the differences by page. The specific presentation of comparison results is based on the actual client-side display. If you need strict left-right contrast or overlay effects, you can combine the general approach in the next section.
- For the scanned document, first use the client's OCR function to recognize the text, then perform text-level comparison. After identification, check paragraph by paragraph, especially numbers, units, personal names, and technical terms.
- When you find content that needs to be changed, directly edit the paragraph text in the editor, or use the blackout feature to cover sensitive information before sharing.
- For differences that need to be confirmed by others, save screenshots of key pages and indicate the old and new versions in the file name or remarks.
- If you want to convert certain content into other formats for continued use, you can export it to Word, Excel, or PowerPoint, then organize it in the corresponding software.
General practice when there is no comparison function
If you don't have a comparison tool on hand, you can also use the following method for visual comparison.
Side-by-side display. Open two PDF windows and place them on the left and right sides of the screen to ensure a unified scaling ratio. Most PDF readers support opening multiple windows simultaneously; manually adjust to the same page size to compare page by page. When the drawings are large, you can first use the same magnification and then enlarge key areas locally.
Synchronized scrolling. If the reader supports synchronous scrolling, both sides will turn pages together when enabled, suitable for files with the same page count and structure. If the number of pages or page heights on both sides differ, synchronization will be off, so switching to manual page turning is more reliable.
Stack contrasts. Export two pages separately as images of the same resolution, create a new layer in the image editing software, place the second page above the first page, and set the layer blending mode to "Difference". The same parts will turn black, while different parts will be visible. Lines and text changes will be very noticeable. This method is especially useful for drawing revisions, but when exporting, ensure consistent resolution, otherwise a large number of "false differences" will occur.
Color annotation. When stacking, you can change one color to red and the other to blue; the overlapping parts will appear gray or dark, with the differences clearly visible. This approach makes it easier to share results with colleagues unfamiliar with technical details.
📊 Core Comparison
| Method | What is suitable to watch? | Limitations |
|---|---|---|
| Side-by-side display | Overall layout, page structure, and page-by-page reading | Slight positional shifts are not easily noticed |
| Synchronized scrolling | Long documents with consistent page counts for continuous reading | Misalignment occurs when the number of pages or page heights differs |
| Stacked contrasts | Drawing revisions, precise changes in lines and annotations | When exporting images, the resolution and aspect ratio must be consistent |
| Text comparison | Text version or recognized scanned content | No changes in pure typography or graphics can be found |
After completion, check each item one by one
After receiving the comparison results, it is recommended to check each item to avoid omissions or misjudgments.
| Inspection items | How to see it | What should you do if there's a problem? |
|---|---|---|
| Page number and page order | Check whether the page numbers and chapter titles of the two documents correspond one-to-one | If misaligned, first align the page numbers, then compare again |
| Is the difference real? | Zoom in on the differences you compare and distinguish between genuine changes and false differences caused by scaling and jagged teeth | Redo it once after unifying the resolution and page size |
| Text recognition results | After scanning the document for OCR, digits, units, proper nouns, and signatures are checked paragraph by segment | If errors are found, correct them directly in the editor, and be sure to double-check key data with the originals |
| Drawing lines and annotations | Zoom in to see key nodes, dimensions, and legends | Take a screenshot of the discrepancy location, mark it, and submit it to the person in charge for confirmation |
| Signature and signature position | Confirm whether the seal is intact and if its position has changed | If you cannot identify it, return to the original or ask the handler for confirmation; do not judge based on guesswork |
Both machine translation and OCR can cause recognition errors, so important content must be verified against the original; conclusions cannot be drawn solely based on comparison results.
❓ Frequently Asked Questions (FAQ)
1. The two PDFs have different page counts—can they still be compared?
Yes, you can. First, find the corresponding chapter or page number, align the pages one by one, and then compare paragraph by paragraph. The extra pages are marked separately as "Added" or "Deleted," making the conclusion clearer.
2. Why does synchronized scrolling screen always don't match?
Most of the time, the two documents have different page heights or page counts, or the content is inserted in the middle of the page. Turning off synchronization, switching to manual page turning, and aligning based on page numbers usually solves the problem.
3. Can the scanned copy be compared with the text content?
Yes, but you need to perform OCR recognition first, turning the text in the image into an optional text layer, and then compare. Recognition results may be incorrect; key paragraphs must be checked verbatim with the original.
4. The two drawings are in image format; how do you overlay and compare them?
You can first convert all images into PDFs or images with the same resolution, then use layer blending mode in image editing software to perform difference comparison. If the two images have different sizes, scale them uniformly first; otherwise, the overlay result will be distorted.
5. What if the colors are too messy after layering and you can't see the differences clearly?
Set two pieces of content to different colors, for example, one red and one blue; overlapping parts will display gray. This way, you only need to look at the colored parts to quickly pinpoint the differences.
📝 Summary
When comparing the layouts and drawings of the two PDFs, the key is to first confirm the file type and page size, then choose to side by side, synchronize scrolling, or overlay according to the purpose. PDF drawing comparison tools are just a means to reach reliable conclusions; aligning page numbers, standardizing resolution, and checking results item by item can be achieved.
