Why Is a Scanned PDF So Large, and What Shrinks It?

What's inside a scanned PDF?

Pictures. A typed PDF stores letters and fonts, but a scan stores every page as one big photo.

• Typed PDFs are small: they say which font to use and where each letter goes.
• Scans store pixels: the scanner can't tell text from a coffee stain, so it records every dot.
• Example: a ten-page contract can be 120 KB typed and 40 MB scanned.
• One page adds up fast: an A4 page at 300 dots per inch (dpi) in color is about 8.7 million pixels. That's about 26 MB of raw image data.

Which settings actually make a scan smaller?

Resolution, color and image compression. Those three decide almost all of the file size.

• Resolution: 300 dpi suits archives and printing. For screens and email, 150 dpi usually looks the same and has a quarter of the pixels.
• Color: a black and white page scanned in color still stores three color channels. Grayscale PDF often takes a big chunk off.
• Compression: Compress PDF decides how much detail to drop from each page image. Strong settings blur small letters.

Why doesn't the usual advice shrink it?

Because it doesn't touch the images, and the images are nearly all of the file.

• Removing metadata: the title and author fields are a few hundred bytes. On a 40 MB scan, you won't notice.
• Deleting a blank page: one page out of forty saves about a fortieth. Remove Blank Pages still helps tidy the file.
• Zipping the PDF: the images are already compressed, so a ZIP saves only a percent or two.
• Running OCR: OCR PDF adds a hidden text layer. That makes the file a little bigger, not smaller.

What order should I try things in?

Start with the change that costs nothing, and leave the one you can't undo until last.

• Go gray first: if the pages are black and white, it cuts the size without blurring anything.
• Then compress: pick a medium level and check the smallest print at 100% zoom.
• Lower the resolution last: you can't undo it without scanning again.
• Size limit: if a form says 10 MB max, Compress to Target Size aims for that number, so you don't have to guess.

Which phone app should I scan with now?

Microsoft Lens is gone, so most people now scan with OneDrive, Apple's Preview or Google Drive. They all save PDFs that are often bigger than they need to be.

• Microsoft Lens: Microsoft stopped new scans in the app on March 9, 2026, and points people to the scanner built into OneDrive.
• iPhone: iOS 26 brought Apple's Preview app to the iPhone, and it scans paper straight to PDF.
• Why phone scans are big: they often keep every page in color at a high resolution, even a plain black and white letter.
• The fix is the same: run the PDF through Grayscale PDF and Compress PDF before you email it.

Try the tools

Try it on your own file. The tools are free and you don't need an account.