A PDF that refuses to attach to an email or gets rejected by an upload form is a common headache. The fix depends on why the file is large in the first place. A 40-page text report and a 5-page scanned contract can be the same size for completely different reasons, and they need different treatment. This guide explains what drives PDF size and how to choose the right way to shrink it.
Why PDFs Get Large
- Images: By far the most common cause. Photos inserted at full camera resolution can be several megabytes each, even if they are displayed as a small thumbnail.
- Scanned pages: A scanned document is essentially one image per page. Scanning in colour at 600 dpi produces very large files.
- Embedded fonts: PDFs embed fonts so they look the same everywhere. Several full font families can add a few megabytes.
- Inefficient structure: Some programs write PDFs with duplicated or loosely packed internal objects, or keep old versions of content after repeated edits.
Two Kinds of Compression
Lossless optimisation
This repackages the PDF's internal structure more efficiently without touching the content. Text, images, and layout stay exactly the same. It works best on files that were saved inefficiently or edited many times. The savings vary: some files shrink noticeably, while others that were already well optimised barely change.
The ToolGhost Compress PDF tool uses this approach. It rebuilds the file with compact object streams, runs entirely in your browser, and never uploads your document. Because it does not re-encode images, the output looks identical to the original.
Lossy compression (downsampling images)
This reduces the resolution and quality of images inside the PDF. It is what produces dramatic reductions on photo-heavy or scanned files, from 30 MB to 3 MB for example, but it permanently lowers image quality. Done carefully, the difference is invisible on screen; done aggressively, text in scans becomes blurry.
Which Approach Fits Your File?
| Type of PDF | What usually works |
|---|---|
| Text documents, reports, invoices | Lossless optimisation, or re-exporting from the original program |
| Documents with a few large photos | Resize the photos in the source document, then export again |
| Scanned paperwork | Re-scan at lower resolution or in greyscale |
| Presentations exported to PDF | Compress images in the presentation software before exporting |
Practical Ways to Shrink a PDF
1. Fix it at the source
If you still have the original Word, Google Docs, or PowerPoint file, this is the most effective option. In Microsoft Word, choose Save As → PDF and select "Minimum size (publishing online)". In PowerPoint and Word you can also use the Compress Pictures option to lower image resolution to 150 ppi, which is plenty for on-screen reading.
2. Re-scan smarter
For paperwork, 200 dpi greyscale is readable for almost all text documents and produces far smaller files than 600 dpi colour. Use colour only for pages where it matters, such as signatures in blue ink or colour charts.
3. Remove what you do not need
Blank pages, duplicate appendices, and cover sheets all add weight. Use Split PDF to extract only the pages you need before sending.
4. Run lossless optimisation
Try the Compress PDF tool. It takes seconds, keeps your file private, and cannot reduce quality. If the file is still too big afterwards, images are almost certainly the cause, and one of the steps above will make the bigger difference.
5. Use your operating system
On a Mac, Preview offers File → Export with the "Reduce File Size" Quartz filter. It is aggressive, so check that text and images are still clear before sending.
Common Size Limits
- Email: Gmail allows attachments up to 25 MB per message, and Outlook's limit is around 20 MB. Remember that attachments grow by roughly a third when emailed because of how they are encoded.
- Job portals and government forms: Often 2–10 MB per file. Check the exact limit before you start.
- Web pages: For downloadable PDFs on a website, smaller files load faster, especially on mobile data.
Checking the Result
- Open the compressed file and zoom to 100% on a page with small text.
- Check any photos, charts, and signatures for blurring.
- Test links and bookmarks if the document relies on them.
- Keep the original until you are sure the compressed version is acceptable.
Frequently Asked Questions
Why did my PDF barely get smaller?
The file was probably already well optimised, and its size comes from images or scanned pages. Lossless optimisation cannot shrink image data; you need to reduce image resolution at the source or re-scan.
Is it safe to compress confidential PDFs online?
It depends on the tool. Many services upload your file to their servers. ToolGhost's Compress PDF processes the file locally in your browser, so the document never leaves your device.
Can I compress a password-protected PDF?
Encrypted PDFs usually need to be unlocked first with the password, using the program that created them.
Will compression remove my digital signature?
Any change to a digitally signed PDF invalidates the signature. Compress documents before they are signed, not after.