Compare PDF Files

When do you need to compare two PDFs?

When you receive a "revised" rental or employment contract and want to know where it changed without rereading everything. Or when you review a thesis chapter a supervisor sent back, or check that the amounts on two similar invoices match. The tool puts matching pages in front of you and marks where they differ.

How the comparison works

The comparison is visual, not textual: every page of both files is rendered as an image at double resolution, then page 1 is compared with page 1, page 2 with page 2, and so on. File 1 is the one you added first; file 2 is the one you added after it. The "Comparison Mode" menu has two choices:

• Visual Diff Overlay (default): the two images are compared pixel by pixel. Unchanged areas appear in gray, content found only in file 1 (removed) appears in red, and content found only in file 2 (added) appears in blue. A replaced word usually shows as red and blue overlapping. The output page has the same size as file 1's page.
• Side-by-Side View: the two pages are placed next to each other on one wide page, each with its file name and page number above it.

If one file is longer, its extra pages are compared against a blank page, so they show up entirely as differences.

How to compare two PDFs

1. 1
   Add the original file first, then the modified one. If you add more than two, only the first two in the list are used.
2. 2
   Under "Comparison Mode", choose "Visual Diff Overlay" or "Side-by-Side View".
3. 3
   Click "Compare Documents Now".
4. 4
   Click "Preview Comparison Report", then "Download Comparison Report" to save it as comparison_report.pdf.

Tips for a useful result

• It works best on two files with the same layout, such as two exports of the same Word document (see Word to PDF).
• If a line is inserted near the top of a page, every line below it shifts and gets flagged. Switch to the side-by-side view and read the difference yourself.
• If a page was inserted mid-document, every later page is compared with the wrong counterpart. Temporarily remove the extra page with Remove Pages and compare again.

Limitations

• It doesn't read text or list added and deleted words; it only shows differences as an image.
• Report pages are images: text in them can't be selected or searched.
• Two separate scans of the same paper never match pixel for pixel, so differences will appear everywhere.
• If page sizes differ (A4 and Letter, for example), file 2's page is scaled to file 1's page width. If it is relatively taller, part of its bottom may be cut off, and differences caused by the size change itself still show up.
• Side-by-side report pages are wide because they hold two pages, so you may need to zoom out in your viewer.
• A file with an open password gives a generic error; open it first with Unlock PDF.

Where is the processing done?

The contracts and documents you compare stay on your device: rendering and comparison happen in the browser, and neither file is transferred to our servers.

Compare PDF: frequently asked questions