How to Compress a PDF Without Losing Quality

· 3 min read

A PDF that is too big to email is one of the most common file problems there is. The good news is that you can often make it much smaller without any visible change. The trick is knowing what is making the file large in the first place.

What actually makes a PDF big?

A PDF is a container. Text takes up remarkably little space — a 50-page report of plain text might be under 500 KB. The weight usually comes from three places:

  • Images. Photos and scanned pages are by far the biggest contributors. A single page scanned at high resolution can be several megabytes.
  • Embedded fonts. PDFs include the fonts they use so they look the same everywhere. Unusual fonts with many characters add weight.
  • Inefficient structure. Some programs save PDFs with redundant objects, uncompressed data or leftovers from earlier edits.

Lossless vs lossy compression

Lossless compression tidies up the structure and packs data more efficiently. Nothing you can see changes: text remains selectable and searchable, and links keep working. The catch is that savings are usually modest — often 5 to 20 percent — and a well-made PDF may not shrink at all.

Lossy compression reduces the quality of images, which is where the big savings are. For a scanned document, re-saving each page as a moderate-quality image can cut the size by 70 percent or more. The trade-off is softer detail and, if whole pages are converted to images, text that can no longer be selected.

Which method should you use?

  • Documents created on a computer (exported from Word, Google Docs or accounting software): start with lossless. These files are usually text-heavy and already fairly compact.
  • Scanned documents and phone photos of paper: use image compression. There is no selectable text to lose, and the savings are large.
  • Brochures and presentations full of photos: try lossless first, then image compression if you still need a smaller file — and check the photos still look acceptable.

Step by step with Felx

  • Open Compress PDF and choose your file.
  • Select Basic for a lossless clean-up, or Recommended for scanned documents.
  • Compare the before and after sizes shown on screen.
  • Download the file and open it to check that small print is still readable.

Felx processes the PDF in your browser, so confidential documents are not uploaded anywhere. If the result is not smaller, Felx says so rather than giving you a “compressed” file that is actually bigger.

Tips for avoiding large PDFs in the first place

  • When scanning, 150 to 200 DPI is enough for most documents. 300 DPI is only needed for fine print or archiving, and 600 DPI is rarely necessary.
  • Scan text documents in greyscale or black and white rather than colour.
  • Resize photos before placing them in a document. A 4000-pixel-wide photo shown 10 cm wide on a page is mostly wasted data — the Image Resizer can fix that.
  • When exporting from Word or similar, look for a “minimum size” or “optimise for online” option.

Common questions

Can every PDF be compressed?

No. A PDF that is mostly text and was saved efficiently may already be as small as it can reasonably get. In that case, splitting it into smaller parts with Split PDF may be the better option.

Will compression affect printing?

Lossless compression has no effect. Image compression at around 150 DPI prints acceptably for everyday documents but may look soft on photos or very small text.