Most email providers cap attachments somewhere around 20-25MB, and a lot of ordinary PDFs blow past that without anyone intending it to happen. A five-page contract becomes 40MB because someone scanned it on a phone. A portfolio PDF with a dozen photos won't attach at all. Here's what's actually driving the file size, and what compressing a PDF does and doesn't fix.
Why a PDF gets huge in the first place
A PDF's size is almost never about the text. Text is tiny — a hundred-page novel as plain text is a few hundred kilobytes. What inflates a PDF is everything else bundled inside it: scanned pages saved as high-resolution images, photos pasted in at full camera resolution, embedded fonts, and sometimes multiple copies of the same image left over from editing.
Scanning apps are the most common culprit. Point a phone camera at a page and the app saves it as a large, uncompressed (or lightly compressed) image, then stacks ten or twenty of those into one PDF. A document that's genuinely a few paragraphs of text ends up as a 30MB file because it's actually a folder of photos wearing a PDF extension.
What a PDF compressor actually does
Compressing a PDF doesn't remove pages or content — it re-encodes the images inside the file at a lower quality or resolution, the same tradeoff you'd make saving a JPEG at 60% quality instead of 100%. Fine detail in photos gets softer, scanned text can get slightly less crisp if you zoom in hard, but at normal reading size the difference is usually hard to notice.
How much a file shrinks depends entirely on what's inside it. A PDF that's mostly scanned photos at high resolution can drop dramatically, sometimes to a fraction of its original size. A PDF that's mostly text with a logo in the header won't shrink much at all, because there was never much to compress — the size was never the problem for that file to begin with. If a PDF barely gets smaller after compressing, that's a sign it was already lean, not a sign the compressor failed.
When compression won't save you
A few situations where shrinking a PDF won't fix the actual problem:
- The file is already a compressed scan. If it was exported from a scanning app that already compressed its images, running it through another compressor has little left to squeeze out.
- You need the file searchable or editable. Compression only touches image data — it won't turn a scanned page into selectable text. That's a different job (text extraction or OCR), not a size problem.
- The content genuinely needs to stay sharp. Architectural drawings, fine print, or anything where someone will zoom in isn't a great candidate for aggressive compression, since that's exactly the detail it trades away.
Merge first, then compress once
If the real situation is "I have twelve separate scanned pages and need to send them as one file," don't scan, merge, and then wonder why the combined file is enormous — that's twelve full-resolution images stacked together. Merge the pages into a single PDF first, then compress the merged file once. Compressing after merging is more efficient than compressing each page individually and combining them, since you're only running the process one time instead of twelve.
This also keeps the order of operations sane: merge to get the structure right (page order, orientation), then compress as the last step before sending, so you're not re-doing either step if you change your mind about page order halfway through.
A quick gut check before you compress
If an attachment is bouncing, open the PDF and ask what's actually in it. Mostly scanned images or photos — compression will likely help a lot. Mostly typed text with one small logo — compression won't do much, and the real fix might be splitting the document into two emails or sharing a link instead of an attachment.
Both tools run in your browser, so files aren't uploaded anywhere to process them. Combine pages with the PDF Merger, then shrink the result with the PDF Compressor before you hit send.