How to reduce PDF file size for email and uploads
By the fullpdftools team · Updated
Quick answer: PDFs are large because of images and scans, so shrinking them means lowering image resolution and quality. Open Compress PDF and choose Recommended for most files, Extreme for very strict limits, or Basic to keep text selectable. Removing unneeded pages and scanning at 150 DPI also helps. Always zoom in on small print afterwards to check it is still readable.
Sooner or later you will hit an upload form that says “maximum file size 5 MB” or an email that bounces because the attachment is too big. Many email providers limit attachments to somewhere around 20–25 MB, and government or job portals are often stricter. Understanding why a PDF is large makes it much easier to shrink it sensibly. Searching for “PDF too large” usually means you have hit an email attachment limit or a form’s upload cap.
Why PDFs get big
- Images. A phone photo or a 600 DPI scan can add several megabytes per page. Images are almost always the main reason.
- Scans. A scanned document is just a stack of page-sized pictures with no real text, so it is far heavier than a typed document of the same length.
- Embedded fonts and duplicated resources. These add weight, but rarely more than a few hundred kilobytes.
A text-only PDF is usually already small — there is little to squeeze out of it.
How compression works
To shrink an image-heavy PDF you have two levers: resolution (how many pixels each page image has) and quality (how strongly the JPEG format compresses it). As a rule of thumb, 300 DPI is for professional printing, 150 DPI is plenty for reading on screen or printing an office document, and 72–100 DPI is fine for quick previews on a phone.
Choosing a level in Compress PDF
Compress PDF offers three levels, and the trade-offs are worth understanding:
- Extreme gives the smallest file. Each page is turned into a low-resolution image, so text is no longer selectable and fine print may look soft.
- Recommended renders pages at a higher resolution and quality. It is the best balance for scans and image-heavy files.
- Basic keeps your text as real, selectable text and only tidies the file structure. Savings are small, but nothing about the content changes.
If the tool can’t make a file smaller, it keeps your original rather than giving you a bigger one.
Other ways to cut size
- Split a large document into parts and send them separately.
- Remove pages you don’t need, such as blank scan sides.
- Re-scan at 150 DPI in black-and-white for text documents, instead of 600 DPI colour.
Always check the result
After compressing, zoom in on the smallest text and on any signature or stamp. If it is still readable, you are done; if not, go one level up. Keep your original file, so you can always start again.
Frequently asked questions
How can I make a PDF smaller for email?
Compress it with the Recommended level, which usually brings image-heavy files under common attachment limits of 20 to 25 MB, and often far lower.
Why can’t I select text after compressing?
The Extreme and Recommended levels turn each page into an optimized image. Choose Basic if the text must stay selectable; it saves less space.
Why didn’t my PDF get smaller?
Text-only PDFs are already compact, so there is little to remove. If compression cannot help, the tool keeps your original file rather than giving you a bigger one.
Does compressing a PDF reduce quality?
Lossy compression lowers image detail. At the Recommended level the difference is hard to see on screen; Extreme can make fine print look soft.
What DPI is best for a small but readable PDF?
About 150 DPI is enough for reading on screen. Use 300 DPI only when the document will be printed professionally.