Why Some PDF Files Cannot Be Compressed Further

Reason 1: The images are already well compressed

Images are where most savings come from. If the PDF’s images were already saved at a reasonable resolution and JPEG quality, re-encoding them gains little. Re-encoding can even make an image larger if its original quality was lower than the new setting. SlimMyFile checks every re-encoded image and keeps the original whenever the new version is not clearly smaller.

Reason 2: There are hardly any images

A PDF exported from a word processor or spreadsheet is mostly text and simple drawings. Text is stored very compactly, so a 40-page report without pictures may be only a few hundred kilobytes and already close to its minimum. The lossless clean-up in SlimMyFile’s Low level may shave off a little, but there is no large block of data to remove.

Reason 3: Fonts make up most of the size

Embedded fonts make a PDF look the same everywhere. Well-made PDFs embed only the characters actually used, but some include complete fonts. Fonts for scripts with very large character sets, such as Chinese, Japanese or Korean, can be several megabytes each.

Reducing a font to only the characters used is called subsetting. It requires rewriting the font program and its references, which SlimMyFile’s browser engine does not do. Desktop tools such as Ghostscript, or the optimisation features of professional PDF editors, can subset fonts.

Reason 4: Complex vector artwork

Maps, engineering drawings, detailed charts and some design files contain huge numbers of vector paths. These are stored as drawing instructions and are usually already compressed losslessly. They cannot be reduced without changing the drawing itself, and turning them into images would destroy their sharpness when zooming.

Reason 5: Image types the browser engine leaves alone

To avoid damaging documents, SlimMyFile only re-encodes images it can process safely. It does not recompress:

• CMYK images, typical of print-ready files from design software
• JBIG2 images, common in black-and-white scans
• JPEG 2000 images
• Images with special colour handling, such as masks, indexed palettes or custom decode ranges

If a scanned PDF uses these formats, its normal modes may achieve little, but scanned document mode can still rebuild the pages as compressed images. See how to compress scanned PDFs for the trade-offs.

Reason 6: The file is protected

Password-protected or encrypted PDFs cannot be modified without the password, and SlimMyFile does not attempt to bypass protection. Digitally signed PDFs can be processed if you confirm, but any change invalidates the signature, so for signed documents the original is usually the version you should keep.

Reason 7: The file is very small already

Every PDF needs some structure: a header, an object index and page descriptions. For a tiny file, rewriting that structure can cost as much as it saves, and the result may be the same size or even slightly larger. When that happens, SlimMyFile tells you no useful reduction was achieved and offers your original file instead.

What you can still try

1. Try the next level up in the PDF compressor, from Low to Balanced, or Balanced to High, if images are involved.
2. Remove pages that are not needed, such as blank pages or appendices.
3. If you created the PDF, re-export it with smaller images, or compress the images before inserting them using the image compressor.
4. For scans only, use scanned document mode at a sensible resolution.
5. If fonts are the problem, use a desktop PDF application that supports font subsetting.

Frequently asked questions