You exported a 10-slide presentation as PDF and ended up with a 45 MB file. You scanned a two-page letter and got a 12 MB PDF. PDF file sizes often feel disproportionate to their content — here is why, and what you can do about it.

What actually lives inside a PDF

A PDF file is essentially an archive of objects: page descriptions, font data, colour profiles, embedded images, metadata, form fields, annotations, and the cross-reference structure that ties it all together. Images are almost always the dominant factor. A single full-page background image at print resolution (300 DPI, A4) contains roughly 35 megapixels of data. A 50-page report with full-page images on each page will be very large regardless of how efficiently the text and vector data are compressed.

Why scanned PDFs are especially large

When you scan a physical document, the scanner captures it as a raster image — typically a high-resolution TIFF or JPEG. A single A4 page scanned at 300 DPI in colour is a 25-megapixel image. Even with JPEG compression, each page can be 2–5 MB. Multiply that by 50 pages and you have a 100–250 MB PDF of what is essentially a short report. The solution is to re-render each page at a lower resolution. For a document primarily read on screen, 150 DPI is entirely sufficient.

The three types of PDF compression

Image resampling reduces the resolution of embedded images — the most powerful technique for image-heavy PDFs, used by PurePDF's compression presets. JPEG quality reduction increases the compression ratio at the cost of more visible artifacts. Font subsetting strips unused glyphs from embedded fonts, effective for text-heavy PDFs but requiring a tool that manipulates the PDF's internal structure rather than re-rendering it.

When compression will not help much

Already-compressed PDFs will see diminishing returns from further compression. Re-encoding an 80% JPEG at 80% again degrades quality but barely changes the file size. PDFs consisting primarily of vector graphics also compress poorly with raster-based approaches, because the vector data is already very space-efficient in its native PDF representation.

Practical recommendations

For PDFs you need to email or upload through a web form, use the Screen preset for the smallest file. For PDFs you will print or review in detail, use the Print preset. For everything else, eBook is the right balance. If the PDF still exceeds your target size, consider splitting it into sections using PurePDF's PDF Splitter.