It's a familiar moment: you attach a PDF to an email, hit send, and get an error saying the attachment is too large. Most email providers cap attachments somewhere between 20-25 MB, and it doesn't take much, a handful of scanned pages, or a report with a few embedded photos, to blow past that.
The fix depends on why the PDF is large in the first place, since a scanned contract and a report full of high-resolution photos need different approaches.
Why PDFs get this large in the first place
A PDF's size is almost always driven by the images inside it, scanned pages are essentially just full-resolution photographs of paper, and a report with several high-quality photos embeds all of that image data directly into the file. Text itself takes up very little space; a 50-page text-only PDF might be under a megabyte, while five scanned pages at high resolution can easily exceed 20 MB on their own.
If it's a scanned document
Scanned pages are usually scanned at a much higher resolution than needed for on-screen reading or even printing. If you have access to the original scanning software, rescanning at a lower DPI (150-200 is usually plenty for readable text) makes an enormous difference. If you only have the final PDF, look for a tool that can recompress the embedded images directly, since that reduces the size without needing to redo the scan.
If it's a document with embedded photos
Here the fix is more direct: reduce the size of the images before they go into the PDF. If you're the one assembling the document (for example, combining photos into a PDF for a report), compress each image first, cutting a photo from a phone camera down to reasonable web-viewing quality routinely shrinks it by 70-90% with no visible difference at normal viewing size, and the resulting PDF shrinks proportionally.
A practical workflow
- If you're building the PDF yourself from photos, compress each image first (a tool set to 75-85% quality works well), then combine them into a PDF, this gives you far more control than compressing after the fact.
- If you already have a large PDF and need to shrink it directly, look for a PDF compression tool that recompresses the embedded images, or split it into smaller parts if the whole document doesn't need to go in one email.
- Check your email provider's actual limit before assuming compression alone will get you there, for very large scanned documents, sending a link to a shared file may be more practical than repeated compression.
- Always keep your original, uncompressed file. Compression is one-directional, you can't get the lost detail back once you've shrunk it.
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