Free PDF Compressor

How to compress a PDF

1. Drop in the PDF.
2. Leave lossless selected for a first attempt.
3. Press Compress PDF and read the before-and-after sizes.
4. If it is still too large and the document is mostly scans, switch to strong mode and try again.

What actually makes a PDF large

Size in a PDF is dominated by images. A scanned page photographed at 600 dpi carries several megabytes of pixels; the same page as typed text is a few kilobytes. So before compressing, work out which kind of document you have — it decides whether compression can help at all.

The usual culprits, in order:

• Scans. Every page is a photograph. Almost always the reason a PDF is enormous.
• Photographs placed at full camera resolution when the page only displays them at 8 cm wide.
• Embedded fonts, especially when a document uses many typefaces and each is embedded in full.
• Structural bloat — revision history, orphaned objects and uncompressed cross-reference tables left by the exporting software.

Only that last category is safely removable, which is what lossless mode does. It rewrites the document's internal structure with object streams and drops what nothing references. On a file exported by a careless tool this can cut a noticeable chunk. On a clean file it saves almost nothing — and the tool will say so rather than pretending.

The trade-off in strong mode

Strong mode renders each page to an image and assembles a new PDF from those images. On a scanned 20 MB document this can be dramatic, because your scanner's uncompressed output is replaced with efficiently compressed JPEG.

You are giving up real things, and you should know which:

• Text stops being text. No selecting, no searching, no copying.
• Screen readers can no longer read the document, which matters if anyone you send it to relies on one.
• Fine detail softens, most visibly on thin lines and small type.
• The file can no longer be usefully converted to Word.

For a scan you are emailing so someone can look at it, that is a fair trade. For a contract someone needs to search, or a document that must remain accessible, it is not.

When compression is the wrong tool

If you are fighting an attachment limit, removing pages often beats compressing them. Ten pages of a hundred-page report is a 90% reduction with no quality loss at all — use Split PDF. Similarly, if the PDF exists only to carry photographs, sending the images themselves is smaller than wrapping them in a document.

And if the document came from Word, re-exporting it from Word with the "minimum size" option frequently beats any after-the-fact compression, because the original software knows what the content is.