You scan four pages for a form, and the result is 28 MB. The portal's limit is 2 MB. The problem is rarely the page count — it is what is inside those pages. Once you know what makes a PDF heavy, shrinking it becomes straightforward.
Reason 1: Scanned Pages Are Photos
When you scan or photograph a document, each page becomes an image. At 300 DPI, a single A4 page holds roughly 8.7 million pixels. Multiply that by the number of pages and colour channels, and the size climbs fast. A typed PDF of the same text might be a few hundred kilobytes.
Fix: scan at 150–200 DPI for on-screen use, use greyscale for plain text documents, then run Compress PDF.
Reason 2: High-Resolution Photos
Phone cameras produce 12–50 megapixel images. When you drop them into a Word document and export to PDF, the full-resolution photo is often kept, even if it appears as a small thumbnail on the page.
Fix: resize images before inserting them, or compress the finished PDF — the "ebook" level brings images down to a sensible on-screen resolution.
Reason 3: Embedded Fonts
Fonts are embedded so the document looks right everywhere. Usually only the characters you used are included, but some programs embed entire font families, and decorative or Urdu/Arabic fonts can be large.
Fix: stick to common fonts where possible, and export with "embed only used characters" (font subsetting) if your software offers it.
Reason 4: Merged Files With Duplicated Resources
Combining many PDFs can repeat the same logo, font or background image dozens of times.
Fix: merge first, then compress the combined file — compression can discard duplicates.
Reason 5: Hidden Extras
Some PDFs carry layers, previous editing history, form data, thumbnails or even attached files. You do not see them, but they add weight.
Fix: exporting a fresh copy ("print to PDF") or compressing usually strips most of this.
Quick Diagnosis Table
| What the PDF contains | Likely size | Best action |
|---|---|---|
| Typed text only | Under 1 MB | Usually nothing needed |
| Text + a few photos | 1–5 MB | Compress at ebook level |
| Scanned pages in colour | 5–50 MB | Rescan at lower DPI or greyscale, then compress |
| Many merged documents | Varies | Compress after merging |
Step-by-Step: Bring a Large PDF Down
- Open Compress PDF and upload the file.
- Choose the ebook level for a good balance of size and clarity.
- Download and check the result by zooming in on text and photos.
- If it is still over the limit, repeat with the screen level, or read why a compressed PDF can still be too big.
For more techniques, see our guide to compressing a PDF without losing quality.
Summary
Large PDFs are almost always large because of images. Control the resolution at the source when you can, and compress the finished file when you cannot.
