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How to Reduce PDF File Size (and Why PDFs Get So Big)

By Antuparthi Manoha Malik PaulPublished 3 October 20266 min read

What makes PDFs large, which compression methods keep text sharp, and how to get under email and portal upload limits.

Upload portals and email often cap attachments at 2, 5 or 10 MB. Here's why PDFs get large and how to shrink them without making them unreadable.

Why PDFs get big

  • Scans and photos. A scanned page is a full image. Scanning at 600 DPI in colour produces enormous files; 200–300 DPI greyscale is usually plenty for documents.
  • High-resolution images placed in documents straight from a phone camera.
  • Embedded fonts — usually small, but full font files in many styles add up.
  • Inefficient structure from some PDF creators: duplicated resources and uncompressed objects.

Text itself is tiny. A 100-page text-only report may be under 1 MB, while five scanned pages can be 20 MB.

Ways to reduce size, from gentlest to strongest

  1. Remove what you don't need. Delete blank or unnecessary pages, or split the document and send only the relevant part.
  2. Lossless optimisation. Rebuilding the file structure keeps every page identical and text selectable. Savings range from nothing to modest, depending on how the PDF was made.
  3. Recompress the images inside. Tools such as Ghostscript downsample and recompress embedded images while keeping text as text. This is the best balance when available.
  4. Re-render every page as an image. This gives the biggest reduction for scans, but text becomes part of the image: it can no longer be selected, searched or read by screen readers.

Fix the source if you can

Prevention beats compression. Scan at 200–300 DPI, in greyscale for black-and-white documents. Resize or compress photos before inserting them into Word. When exporting from Word or PowerPoint, choose a “minimum size” or “standard” option rather than “high quality print”.

Check before you submit

After compressing, zoom in on the smallest text and any signatures or stamps. Many portals reject documents they can't read, which is worse than a slightly larger file.

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