A PDF gets large for one main reason: embedded images. A ten-page document that is mostly text might be a few hundred kilobytes, but the moment you scan paper pages, paste in screenshots, or drop in high-resolution photos, those images are stored at full quality inside the file. A single scanned page can weigh 3–5 MB on its own, so a 30-page scan easily becomes a 100 MB monster. Fonts, embedded colour profiles, and duplicated image data add a little more on top.
How PDF compression works
Compression re-encodes the images inside your PDF at a lower resolution and a slightly lower JPEG quality, then strips out redundant data the viewer never needed. Crucially, the text stays untouched. Text in a PDF is stored as vectors and font glyphs, not pixels, so it remains perfectly crisp no matter how hard you compress. That is the key insight most people miss: compression trades away photographic detail, never the sharpness of your letters. If your document is pure text, there is very little to compress — and that is normal, not a fault.
Choosing the right compression level
- Maximum compression — the smallest possible file, with visibly softer images. Perfect for emailing a scanned contract, attaching a receipt, or uploading to a portal with a tight size cap. Expect 60–90% smaller on image-heavy files.
- Balanced — our recommended default. Images stay clearly legible while the file shrinks 40–70%. This is the right choice for almost everything you share day to day.
- High quality — keeps print-grade detail and only trims the obvious waste. Use it when the PDF will be printed, sent to a designer, or archived, and you can accept a more modest 20–50% reduction.
Real-world size limits worth knowing
Most rejections happen because a service has a hard attachment limit. Gmail caps attachments at 25 MB, Outlook.com at around 20 MB, and many government and banking upload forms sit between 2 MB and 10 MB. If your file is being bounced, compress to Balanced first; if it is still too big, drop to Maximum. Knowing the target number means you rarely have to guess.
Tips to get a smaller file without ruining it
- Scan documents in greyscale or black-and-white rather than full colour when the content is text — colour scans are several times larger.
- If you only need a few pages, split the PDF first and compress just those pages.
- Already-optimised PDFs (exported from Word or a design tool) may barely shrink — that simply means there was no wasted data to remove.
What compression will not do
Compression makes a file smaller; it does not edit, redact, or password-protect it, and it cannot recover quality that was never in the original. It also will not magically shrink a text-only PDF that is already small. Your uploaded file and the compressed copy are processed over an encrypted connection and deleted from our server immediately after you download — nothing is stored, scanned, or shared.