Compare Two PDFs by the details stored inside them

Can I see what changed in the text of two PDFs?

Not with this tool. It does not read the wording, the images, form values, comments or page order. Two contracts with different payment terms but the same file details show no differences. To compare wording, extract the text of both files and paste it into a text diff tool, or use Acrobat Pro's Compare Files.

Why do two copies of the same document show differences?

Saving a file again, or running it through a converter or print-to-PDF step, changes its details even when nothing on the page changed. In our test, a converted copy named a different program as its maker, got new dates and grew in size. A newer creation date belongs to the copy, not to the content.

Are the two PDFs stored?

No. Both files are read during the upload and thrown away with it. Only the two file names, their sizes and the fifteen values are saved as your result. Results made without an account are deleted after 90 days. Results made while signed in stay until the account is deleted.

The fifteen fields that are compared

Each file is read with the full metadata parser and reduced to the fields below. A field is marked Different when the two values are not exactly identical. Matching is literal: a trailing space, a change of capital letters or a timestamp one second apart counts as a difference.

Compared fields and where each value comes from

What is not compared

The comparison does not read the wording on the pages, the pixels of images, form field values, comments or page order. Two contracts with different payment terms but identical properties are reported as having no differences. To check the wording, extract the text of both files and compare it with a text diff tool.

Measured: an original and a rewritten copy

We compared a one-page test file labelled as a Word export with a copy produced by passing it through Ghostscript. Nothing on the page was edited.

Reading the differences

The example shows the pattern left by a converter or print-to-PDF step. The Creator survived because Ghostscript copied it, but the Producer names the new writer and both dates were reset to the moment of conversion. The newer creation date belongs to the copy, not to the content. When one version of a document has a Producer different from the application its Creator names, a conversion step is the usual explanation.

Common causes for each kind of difference

When "no differences" is still not identical

• Matching file sizes and properties make it likely, but not certain, that the files are the same. Comparing checksums such as SHA-256 is the way to confirm byte-for-byte identity.
• Editors that preserve metadata, or a deliberate edit of the date fields, can make two different documents look the same here.
• A field that could not be read in either file is empty on both sides and counts as equal, so missing data hides differences rather than revealing them.

Saving and sharing a comparison

The result page is stored with both file names and the extracted values; the PDFs themselves are not kept. It is private to the browser session or account that created it, which can return to it through its unique link.

Comparing with Acrobat, pdfinfo or a checksum instead

Acrobat Pro's Compare Files reads the page content and marks changed words, moved paragraphs and replaced images side by side. When the question is what changed in the wording, that is the right tool, and a property comparison is no substitute for it. Free readers such as Preview and the Chrome viewer have no compare feature at all, which is one reason this page exists.

On a computer with Poppler installed, pdfinfo prints most of the fields this tool compares, and a text diff shows the differences: pdfinfo a.pdf > a.txt && pdfinfo b.pdf > b.txt && diff a.txt b.txt. For the narrower question of whether two files are the exact same bytes, a checksum settles it faster than any field list: matching SHA-256 values mean identical files, and a single changed byte changes the value.

PDFCheck fits the case in between: two attachments from an email thread, a few seconds to learn whether one was re-saved, converted or re-dated, no software on the machine, and a result page you can return to from the same browser.

Before you compare two files

• Can I send someone the result link? Not usefully. Every result is saved as private, and only the browser session that created it can open the page; anyone else sees a 404. Copy the values or take a screenshot to share the finding.
• Does a comparison use one check or two? Neither, in the current code. The upload is blocked once your daily checks are used up, but a comparison is counted in its own tally and does not reduce the daily total. Each file must still be within your plan upload limit, 50 MB on Free and 100 MB on Pro and AI.
• What does "Comparison failed" mean? The message is followed by the parser's own error, and the usual cause is a damaged file the metadata reader cannot open. An encrypted file does not fail: the parser skips decryption, so the Encrypted row says Yes, and text fields it could not read come out empty on that side.