เปรียบเทียบไฟล์ PDF

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This compares two PDFs twice and reports the results separately: a word-level diff of what the documents say, and a page-by-page pixel comparison of what they look like. What you get is an HTML report naming the changes, not a blended similarity score. If the two files turn out to be identical, the report says so in its title rather than leaving you to interpret an empty list.

When a document has come back and you need to know what moved: a contract returned by the other side, a specification revised by a supplier, a design proof against the version you approved, or two copies of a report you are no longer certain are the same file.

วิธีทำงาน

1. Drop both PDFs onto this page. The first is treated as the original and the second as the revision, which is what the labels in the report mean.
2. Choose text, appearance or both. Both is the default and costs one render of each document.
3. For the appearance pass, set the resolution and the sensitivity - sensitivity is how large a pixel difference has to be before it counts as a change rather than as rendering noise.
4. Leave ignore whitespace on so that a reflowed paragraph does not report as a change on every line.
5. Press Compare and open the report. It lists the added and removed passages and the percentage of pixels that differ on each page.

A lawyer wants the words and a designer wants the pixels, and one blended percentage would answer neither. The text pass diffs the documents word by word, so a changed sentence reports the words that changed instead of a scatter of letters. The appearance pass renders both documents and compares page one with page one, counting the pixels that differ beyond the tolerance you set.

The text comparison treats each document as a single stream of words, so it tells you what changed rather than which page it changed on. That is deliberate - a paragraph inserted on page two would otherwise make every later page report as different - but it means the appearance pass is where you look when you need location.

The appearance pass compares by page number, which is right for a revision and misleading for an insertion: add a page at the front and everything after it differs from its counterpart. Pages present in only one document are flagged as such, and two documents on different paper sizes are compared over the region they share, with the rest counted as different.

Two renders of genuinely identical pages are not bit-identical, because anti-aliasing is not deterministic to the last channel value, so a comparison with no tolerance reports that noise as change. The default sensitivity of 20 per cent ignores small differences. The report also lists at most 200 individual changes and then says how many it has not listed, rather than truncating quietly.