You need to email a contract, upload a CV to a job portal or submit a form to a government website, and the PDF is 38 MB. The portal says 10 MB maximum. Compressing a PDF is usually easy once you know what makes it big in the first place. This guide explains what’s inside a PDF, how compression works, the size limits you’ll meet, and when splitting or merging is a better fix. Our PDF compressor runs in your browser, so the file doesn’t leave your device.
Why PDFs get large
A PDF is a container. Its size depends on what’s inside:
| Content | Typical size impact |
|---|---|
| Text | Tiny: a 100-page text-only document is often under 1 MB |
| Vector graphics (charts, logos) | Small |
| Embedded fonts | Moderate; full font sets can add several hundred KB each |
| Images and scans | Large: usually 80–99% of the file |
| Hidden extras | Thumbnails, metadata, form data, earlier revisions, attachments |
A single smartphone photo can be 3–5 MB. Put 15 of them in a PDF without resizing and you have a 60 MB file. Scanned documents are essentially one image per page, so a 20-page colour scan at 600 dpi is huge.
How PDF compression works
Good compression tools combine several techniques:
- Downsampling images: reducing image resolution to what’s actually needed. A photo at 300 ppi displayed on screen gains nothing over 150 ppi.
- Recompressing images: re-encoding photos as JPEG at a moderate quality (for example 70–80%), and black-and-white scans with efficient formats such as CCITT G4 or JBIG2.
- Converting colour to greyscale for documents that don’t need colour.
- Subsetting fonts: embedding only the characters used.
- Removing unused objects: old revisions, thumbnails, metadata and duplicate resources.
- Object stream compression: packing the PDF’s internal structure more efficiently.
Choosing a level
| Purpose | Image resolution | Expected result |
|---|---|---|
| Email, web, on-screen reading | 100–150 ppi | Smallest file, text still crisp |
| General office use | 150–200 ppi | Good balance |
| Printing | 300 ppi | Larger, best quality |
Text that is real text (not a picture of text) stays perfectly sharp at any compression level.
Example
A 24-page scanned colour report at 300 dpi is 42 MB. Converting pages to greyscale and compressing images to 150 dpi at 75% JPEG quality brings it to about 4–6 MB, a reduction of roughly 85–90%. Exact results depend on the content.
Size limits you’ll run into
| Service | Limit |
|---|---|
| Gmail | 25 MB attachments per email (sending); larger files become Google Drive links |
| Outlook.com | 25 MB per message; OneDrive links for larger files |
| Microsoft 365 / Exchange business accounts | Set by the administrator; often 20–35 MB, sometimes lower |
| Many job portals and government upload forms | Often 2–10 MB per file |
Email attachments are encoded in Base64 for transport, which adds about 33%. So a 20 MB PDF can become roughly 27 MB in transit and be rejected even though the limit says 25 MB. Keep attachments to about 18 MB or less to be safe. Our Base64 guide explains why.
Merge or split instead?
Sometimes the answer isn’t compression:
- Too big to upload as one file? Split the PDF into parts (for example, by chapter or by page range) and send them separately.
- Only need a few pages? Extract just those pages; a 2-page extract from a 200-page scan is tiny.
- Several separate scans or files? Merge them into one PDF, then compress once, so fonts and resources are shared.
- Very large files to share? Use a cloud storage link (OneDrive, Google Drive, Dropbox) instead of an attachment.
Making better PDFs from the start
- Export, don’t print. “Save as PDF” or “Export to PDF” from Word, Google Docs or Pages keeps text as text. Printing to an image-based PDF bloats the file.
- Resize photos first. For a report, 1,600 px on the long side is plenty. See our image optimisation guide.
- Scan sensibly. 200–300 dpi greyscale is ideal for most documents; use black and white for plain text; reserve colour for photos and signatures that must show colour.
- Use OCR on scans so text is searchable and selectable, which also helps accessibility. Our scan to PDF guide covers this.
Privacy and security
- Browser-based tools process the file locally; nothing is uploaded.
- Uploading tools send your document to a server. Avoid them for medical, legal, financial or identity documents unless you trust the provider.
- Compression may remove some features, such as digital signatures (re-saving invalidates them), form fields or embedded attachments. Compress before signing, not after.
- Password-protected PDFs usually need to be unlocked before they can be compressed.
Common mistakes
- Compressing to the lowest quality and producing unreadable scans of small print.
- Compressing a digitally signed contract and breaking the signature.
- Converting a text PDF to images (for example, by “printing” it) and making it larger.
- Repeatedly compressing the same file, which degrades JPEG images further each time.
- Forgetting the Base64 overhead when emailing.
Checklist
- Find out the size limit you need to meet.
- Compress images to 150 ppi for screen, 300 ppi for print.
- Use greyscale for text documents that don’t need colour.
- Split, extract or share a link if the file is still too big.
- Compress before signing, and keep the original.
- Use a browser-based tool for sensitive documents.