You attach a PDF to an email and get bounced back: file too large. It's a 12-page document — how is it 40 MB? The answer is almost always images, and understanding that is the key to compressing it without turning your document into a blurry mess.
Why PDFs get so big
A PDF is a container. Text costs almost nothing — a page of plain text might be a few kilobytes. Images are the opposite. A single photo from a modern phone camera can be 4–8 MB on its own, and a scanner set to 600 DPI produces an image file per page that dwarfs anything the text contributes.
This is why a 200-page text report can be smaller than a 3-page scanned contract. If your PDF is unexpectedly large, it's holding image data at far higher resolution than anyone viewing it will ever need.
The two ways compression actually works
- Re-compressing the images inside the PDF — the file keeps its structure, text stays selectable, and only the embedded images are re-encoded at a lower quality or resolution. This is the ideal approach but requires tooling that can rewrite the PDF internals.
- Re-rendering each page as an image — every page is drawn and saved as a compressed picture, then reassembled into a new PDF. This shrinks scans dramatically and works reliably in a browser, but it flattens the page: text becomes part of the image.
Most free browser-based compressors, including ours, use the second method. That's not a flaw to hide — it's a trade-off worth knowing before you compress something you'll need to edit later.
What compression can and cannot fix
Compression helps most when your PDF is dominated by scans or photographs. It helps least when the file is already mostly text, because there simply isn't much to squeeze — a text-heavy PDF is already close to as small as it usefully gets, and re-rendering it as images can even make it larger.
If a plain-text PDF is still too big, the culprit is usually embedded fonts or an oversized logo image on every page, not the text itself.
Practical rules of thumb
- Compress a copy, never your only original — flattening is not reversible.
- Start with the middle setting. Go to maximum only if you're still over the limit; the quality drop is visible on fine print.
- If the result barely shrinks, your PDF was already efficient. Look for a different fix, like splitting it into parts.
- If you need the text to stay searchable or editable, keep the uncompressed original alongside the compressed one for sending.
Common size limits worth knowing
Most email providers cap attachments around 20–25 MB. Government and job-application portals are often stricter, sometimes as low as 2–5 MB per file. If you're repeatedly fighting a limit, consider whether splitting the document is a cleaner solution than compressing it into illegibility.