Compare PDF Online — Side-by-Side & Change Report

What Does the Compare PDF Tool Produce?

The output is a single ZIP file containing two PDFs. The first, side-by-side.pdf, places the original document on the left and the modified version on the right in a two-column layout — deletions are highlighted in red on the left, additions in green on the right, and unchanged lines appear in muted grey on both sides. The second, change-report.pdf, is a unified change log with summary statistics (total additions, deletions, and unchanged lines) followed by every modified line in a familiar redline format. Together, the two PDFs give you both an at-a-glance visual comparison and a structured audit log you can keep on file.

How Are Differences Detected?

PdfDocShift extracts the text from both PDFs with whitespace normalisation, then aligns the resulting line arrays using a longest-common-subsequence (LCS) diff algorithm. LCS is the same algorithm used by git diff and the diff command-line utility — it finds the optimal alignment so that genuinely matching lines stay paired and real edits show up as discrete additions or deletions, rather than as cascading mismatches caused by a single inserted line.

When Should You Use PDF Comparison?

PDF comparison is essential whenever you need to verify what changed between two versions of a document. Common use cases include contract redlines — confirming exactly what was amended between drafts before signing; policy review — auditing changes between published versions of company policies; research and publishing — comparing manuscript revisions against an editor's copy; regulatory filings — verifying that only the requested changes were made between submissions; and technical documentation — tracking changes between releases of a specification or manual. Without a comparison tool, finding all differences manually is error-prone and slow, especially across documents with hundreds of pages.

Comparing Scanned PDFs vs. Text-Based PDFs

Comparing two text-based PDFs (PDFs created digitally, such as those exported from Word, Google Docs, or InDesign) is fast and accurate because the text is stored as searchable characters. Comparing two scanned PDFs — where each page is an image — is not directly possible: there is no text to extract. To compare scanned files, first run both through the OCR PDF tool to add a searchable text layer, then upload the OCR'd files here. OCR introduces some character-level noise, so for the cleanest comparisons start from text-based originals where possible.

Privacy and Retention

Both files are uploaded over 256-bit SSL to encrypted cloud storage tied to your anonymous browser session. They are accessible only to that session for the duration of processing and the optional retention window, after which they are permanently deleted within 2 hours. We never read your documents, share them with third parties, or train machine-learning models on them. There is no account, no registration, and no log retention beyond what is needed to operate the service.