How to compress a PDF to meet an upload limit

What makes a PDF large

Almost always, pictures. Text is stored as characters and fonts, which take very little space: a 30-page report typed in a word processor can be a few hundred kilobytes. A scanned page is the opposite - the whole page is one photograph, and a photograph of a sheet of paper at office-scanner resolution can be one or two megabytes on its own.

So a ten-page scan from a phone or a copier easily reaches 10 to 20 MB, and a document with many photos - a property listing, a product catalogue, a portfolio - can be far larger. That is when upload forms start saying no.

Common limits are 1 or 2 MB on university, visa and government portals, a few megabytes on job sites, and around 25 MB for email attachments (Gmail, for example). A good compressor brings a scan under those limits while keeping it easy to read.

Step by step: compress a PDF to a target size

1. Open Compress PDF and select your PDF, or drop it onto the box. One PDF at a time, up to 50 MB.
2. Choose Target size and enter the limit, such as 1 MB or 2 MB. If you don't have an exact limit, pick a level instead (see below).
3. Press Compress PDF. The result shows the new size and how much smaller it is.
4. Download it, then open it once and zoom into a picture or a scanned signature to check it is still clear enough for the reader.

The work happens in your browser: the PDF is read from your device and the smaller copy is written there too, so confidential documents are never uploaded anywhere. The tool counts 1 MB as 1,000,000 bytes, so the result is under the limit however the website measures it.

Light, Medium or Strong: choosing a level

When there's no exact limit to hit, a level is quicker:

Whichever you choose, only pictures are touched. Text, fonts, lines and drawings are kept exactly as they were, so typed text stays perfectly sharp and you can still search and copy it.

When a PDF won't get smaller

Sometimes the result is barely smaller, or the tool tells you the PDF couldn't be made smaller. That is honest, not a fault - it means there's little left to remove:

• The PDF is mostly text. It is already as compact as it can be; there are no big pictures to shrink.
• The pictures are in types the tool leaves alone: black-and-white scans stored as JBIG2 or CCITT (already extremely compact), and print-colour CMYK images.
• It was compressed before. Compressing an already-compressed PDF again gains little and costs quality.
• The size comes from embedded fonts. Some PDFs carry complete font files; compression doesn't change those.

If a target size can't be reached without making pictures unreadable, you get the smallest readable version and a message saying so - rather than a PDF nobody can read. In that case, the next section has better options.

Other ways to get under the limit

• Remove pages the reader doesn't need. Blank backs of scanned pages, duplicates and cover sheets all add weight. Delete them in Organize PDF first, then compress.
• Split the document. If the portal accepts several files, Split PDF can cut a long scan into parts that each fit.
• Scan it smaller in the first place. For documents read on screen, 150-200 DPI in greyscale is plenty; 600 DPI colour only makes huge files. Scan to PDF can aim for a size directly: choose an option such as Under 1 MB in File size before you create the PDF.
• Make PDFs from photos at a lower quality. In JPG to PDF, choose Medium under Image quality instead of Original.

Compressing confidential documents safely

The PDFs people most often need to shrink are exactly the ones they shouldn't hand to a stranger's server: bank statements, payslips, contracts, passports and medical letters. Most online compressors upload the file, process it remotely and keep it for some time afterwards.

With SentrueFile the file never leaves your device. The page you open contains the compressor; your browser reads the PDF from your disk, re-compresses it in memory and hands the result back as a download. When you press Start over or close the tab, everything is cleared. See how files stay private for the details.

One limitation follows from that: a password-protected or restricted PDF can't be compressed, because the tool can't change it. Remove the protection in the program that created the file, then compress the unlocked copy.

How to check a PDF's size before you upload it

Upload forms often reject a file only after you've filled everything in, so it pays to check the size first. Every system shows it in a slightly different place:

• Windows: right-click the file and choose Properties; the size is on the General tab.
• Mac: select the file and press Command-I (Get Info), or turn on the size column in Finder's list view.
• Android: in the Files app, long-press the file and open its details or info.
• iPhone and iPad: in the Files app, long-press the file and choose Get Info.

Compress PDF also shows the new size and the percentage saved as soon as it finishes, so you can see straight away whether the result fits before you download it.

A note on units: websites don't always agree on what a megabyte is. Some count 1 MB as 1,000,000 bytes, others as 1,048,576. A file of 1,020,000 bytes passes the second rule and fails the first. Aiming a little below the limit - or using Target size, which stays at or under 1,000,000 bytes for 1 MB - avoids the problem entirely.