How to use the compare pdf
- Choose the original PDF in the first box and the changed PDF in the second. Both stay on your device. Then press Compare the PDFs.
- Read the four numbers at the top: how many pages each file has, how many lines of text were removed or added, and how many pages look different when drawn.
- Look through Text changes for each removed or added line with its page number. Tick Ignore extra spaces or Ignore capital letters to hide differences that are only spacing or case. The list updates at once.
- Under Page by page, press Show on a page to see it drawn with the changed pixels in red. Use Copy text changes or Download .txt to keep the list.
An invoice with one corrected amount
Say page 1 of the original reads Invoice, Total due: 100, Thank you, and page 2 reads Terms apply. In the new version the total is 120, a line called New clause is added to page 2, and the line Thank you becomes Thank You.
With capital letters counted, the comparison finds 2 lines removed (Total due: 100 and Thank you) and 3 lines added (Total due: 120, Thank You and New clause). With Ignore capital letters ticked, that falls to 1 removed and 2 added. Both pages are listed as different in appearance, since the pixels that changed are drawn in red.
How the text is compared
Each PDF is read page by page, the text is split into lines, and blank lines are dropped. The two lists of lines are then matched with a standard shortest-edit-script method, the same idea used by file comparison tools. Lines that appear in both lists in the same order are unchanged, and the rest are marked removed or added. A line that was edited shows up as one removed line and one added line.
The lines come from the text stored in the file, in the order the file stores it. A change in layout that moves text without changing it, such as a reflowed paragraph, can look like removed and added lines, and tables or multi-column pages may be read in an order that differs from what you see. When more than 1,000 lines differ, the line list is not shown because it would not help, but the page summary still works.
How the pages are compared
Each page, up to the first 50, is drawn at the same width in both files, and every pixel is checked. A pixel counts as changed when any of its red, green or blue values differs by more than 24 out of 255, which hides tiny differences from smoothing. Pages whose heights differ by more than about 1 percent are reported as having a different page size and are not compared. This is the only way to see changes in scanned pages, images and drawings, which have no text.
Two PDFs that print the same can still differ in the file, for example in metadata, fonts or form fields, and none of that is compared here. To check or edit details like the title and author, use the PDF metadata editor, and to join documents after reviewing them, use merge PDF.
Privacy and limits
Both files are opened and compared in your browser. They are not uploaded. Each file can be up to 25 MB and 500 pages. Password protected PDFs cannot be opened, and the tool does not try to remove a password. Very large or complex pages take longer to draw, and you can cancel while the comparison runs.
Assumptions and limits
- The text comparison works on text stored in the PDF. Scanned pages have none, and no text recognition is done.
- Pages are matched by number: page 3 of one file is compared with page 3 of the other. If a page was inserted, later pages will show as different.
- The page comparison covers the first 50 pages. Later pages are still compared as text.
- A pixel counts as changed when a color channel differs by more than 24 out of 255. That limit is a setting of this tool.
Frequently asked questions
Why does an edited line show as one removed and one added line?
The comparison works on whole lines. A line that changed is no longer the same line, so the old version is listed as removed and the new version as added, next to each other.
Can it compare scanned PDFs?
Only by appearance. Scans have no stored text, so the text list is empty, but the page by page view still shows which pages differ and where.
Why is a page marked Different page size?
The two pages are not the same shape, so their pixels cannot be matched. Compare the text instead, or save both files at the same page size first.
Are my PDFs uploaded?
No. Both files are read and compared in your browser tab, and nothing is sent anywhere.