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What is PDF compression?

PDF compression is reducing a PDF's file size so it's easier to email, upload or store. Most of the savings come from images: they're resized to a lower resolution and re-compressed, while text and vector graphics are stored efficiently and stay sharp. Stronger compression gives smaller files but less detailed pictures.

Also called: Compress PDF, Reduce PDF file size

Updated October 14, 2026 · By the PDFZyro team

Why some PDFs are so large

A page of text in a PDF often takes only a few kilobytes. Large files are usually caused by images: scans saved at high resolution or in colour, photos straight from a camera or phone, and pictures pasted into a document at full size. Embedded fonts, many pages, attachments and leftovers of earlier saves add to the size too.

The difference can be large: a 20-page scan saved in colour at a high resolution can take tens of megabytes, while the same pages as a text PDF may be well under one megabyte. Many email services limit attachments to around 20 to 25 MB, which is often why people need to compress a file.

Lossy and lossless compression

Lossless compression stores the same data more efficiently, so nothing changes; PDFs already use it for text and drawings. Lossy compression, such as JPEG for photos, throws away detail the eye is unlikely to notice and gives much bigger savings. Resolution matters most: around 300 DPI suits printing, 150 DPI is fine on screen and for most sharing, and 72 DPI gives the smallest files for viewing on screens only.

Getting a good result

Start with a lighter level and check the pages, especially photos, small print and signatures. If the file is still too large, remove pages you don't need, split it into parts, or remove attachments and hidden data. Keep the original: compression can't bring back detail once it's removed.

Optimizing a PDF for fast web view, also called linearizing, is different: it reorders the file so the first page shows while the rest downloads, without necessarily making it smaller.

How to compress a PDF in PDFZyro

  1. Open Compress PDF and choose one document, or up to 20 at once.
  2. Pick Best quality (pictures up to 220 DPI), Balanced (150 DPI, recommended) or Smallest file (72 DPI).
  3. See the size before and after. The smaller PDF is saved as a new version and the full-quality original stays in its history; if a file can't be made smaller, it's left as it is.
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