A PDF that was "just a couple of pages" can balloon to 100MB or more, and then bounce off every email server you try to send it through. The five usual reasons, in order of how common they are:
1. The PDF is a scan, not a real document
The single biggest cause. A scanned page is one full-page image at 300-600dpi, often saved as uncompressed bitmap inside the PDF. A single scanned page can weigh 5-10MB; a 20-page scanned contract lands around 100MB.
This is the case where compression helps the most. A 600dpi scan of a black-and-white contract can typically shrink 90% with no visible loss. A 300dpi color brochure shrinks 50-70%. The PDF size check tool tells you how much your specific file would save — without uploading it.
2. The PDF embeds high-resolution images
A brochure, catalog, or photo-heavy report usually has 2-5MB of images per page. A 30-page catalog with embedded images lands at 60-150MB. The fix is the same: compress with the ebook quality preset, which downsamples images to 150dpi (screen-readable, light on bytes).
3. The PDF was exported from PowerPoint or Keynote with embedded fonts
Slides exporters sometimes embed every font in the deck "to be safe." Each font can be 1-5MB, and a 20-slide deck with three custom fonts can hit 50MB before any content is added. The fix is to export the PDF without embedded fonts (most slide tools have a "use system fonts" toggle) and then compress if it's still too big.
4. The PDF has a lot of pages
A 500-page reference manual with light content can be 40-60MB. There is nothing wrong with the file — it just has too many pages for email. The right fix is splitting it into chapters and sending each chapter as its own attachment. The PDF size check tool automatically suggests split instead of compress when it sees this pattern.
5. The PDF has invisible layers, annotations, or form fields
A PDF can hold layers and form fields that don't show up when you open it. A 20-page form with hundreds of fields can be 30MB even though it looks like a short document. The fix is to export a flattened version (the compress tool flattens by default), which removes the editable overhead but keeps the content readable.
How to find out which one applies to your PDF
Open the PDF size check, drop the file, and look at the verdict. It reports the size, page count, and an estimated savings percentage — without uploading the file. If the savings estimate is 70% or higher, you almost certainly have a scanned or image-heavy PDF and compress will fix it. If the savings estimate is under 20%, the file is mostly text and you should split it into chapters.
What not to do
- Don't re-save from a screenshot. Re-printing the PDF as a screenshot of itself doesn't shrink it — it adds another layer of compression artifacts and usually makes the file larger.
- Don't convert to JPG and back. A PDF → JPG → PDF round trip destroys the text layer and makes the file image-only. The result is harder to read and often larger, not smaller.
- Don't use a free online compressor that asks for your email. Most of them bundle the upload into a marketing pipeline. DocShop runs the conversion on an isolated worker, deletes everything within an hour, and never reads your content. The original filename is not stored in your history (we keep a short hash) and is never put in the email — the download link still serves the file with its real name.