Why PDFs get so large
A PDF is a container. Text is tiny, but everything else adds up quickly. The usual causes of a bloated file are:
- High-resolution photos or scans, often 300 DPI or more per page
- Scanned documents stored as full-colour images instead of black and white
- Embedded fonts, especially several full font families
- Images pasted in multiple times, or pages exported at print quality
- Extra data such as metadata, thumbnails and edit history
Scanned documents are the worst offenders. A 20-page scan can easily reach 50 MB even though the content is just text on paper.
What compression actually changes
Compression tools reduce size by trading away detail you are unlikely to notice. The main techniques are:
- Downsampling images to a lower resolution
- Re-encoding images with stronger JPEG compression
- Removing unused objects and duplicate data
- Dropping metadata and embedded thumbnails
The trade-off to know about
Some methods re-draw every page as an image. That gives the biggest size reduction, but the text may stop being selectable or searchable. If you need to copy text or search the document afterwards, choose a lighter level, or keep the original alongside the compressed copy.
Always keep your original file. Compression is a one-way change, and you can't restore detail that has been removed.
How to compress a PDF in your browser
- Open the PDF compressor and choose your file. It is opened locally and never uploaded.
- Pick a compression level. Start with the lightest option that meets your size target.
- Run the compression and watch the progress indicator.
- Check the result size and scroll through the pages to confirm it is still readable.
- Download the compressed file. If it is still too large, repeat with a stronger level.
Because processing happens on your own device, large files depend on your computer's memory rather than an upload speed.
Common size limits and what to aim for
Different services have different limits. These are typical targets worth knowing:
- Email attachments: most providers cap attachments at around 20 to 25 MB, including Gmail
- Job and government portals: often between 1 MB and 5 MB per file
- Messaging apps: commonly well above email, but slow on mobile data
If a portal asks for under 1 MB, a scanned document usually needs a strong level, while a text-based PDF often shrinks enough with a light one.
Tips for smaller PDFs from the start
- Scan in greyscale or black and white when colour isn't needed
- Scan at 150 to 200 DPI for ordinary documents instead of 600
- Export from design tools using a web or screen quality preset
- Remove pages you don't need before compressing
Frequently asked questions
Will compressing a PDF reduce its quality?
Usually a little, because images are stored in a lower resolution. A light compression level is rarely noticeable on screen. A strong level can make small text and photos look softer.
Is it safe to compress a confidential PDF online?
It depends on the tool. Many sites upload your file to a server. A browser-based tool processes it on your device, so the file never leaves your computer.
Why is my PDF still large after compressing?
If the file is mostly high-resolution scans, it may need a stronger level. Splitting it into smaller parts or rescanning at a lower DPI also helps.
Compress a PDF in seconds
No upload, no account. Your file never leaves your device.