How to Compress a PDF and Reduce File Size Online

A PDF that is too big to email is a common problem, and a lot of advice about it overpromises. This guide explains what actually makes PDFs large, what our Compress PDF tool does at each setting, and what to do when a file still will not fit.

Why PDFs get large

Understanding the cause tells you the fix. A PDF is a container. Text takes very little room, so a 30-page report with only text may be a few hundred kilobytes. What makes files heavy is almost always images: photographs, scanned pages (every scanned page is one big picture), and screenshots. A scan of ten pages at high resolution can easily reach 20 MB, while the same ten pages typed out would be a fraction of one megabyte.

Fonts embedded in the file add some weight, and some programs write files in a wasteful way, but for most people the large file is large because of pictures.

What the Compress PDF tool does

We want to be exact about this, because many online tools imply more than they deliver. Our Compress PDF tool goes after the part that matters: the pictures. It finds every photo and scanned image inside the file, re-encodes it at a sensible resolution and quality, removes data the file does not need, and rebuilds the internal structure. Text, fonts and vector drawings are left as they are, so text stays sharp and selectable.

You choose how hard it works:

• Recommended – good quality and good compression. Images are brought down to about 150 dpi, which is sharp on screens and fine for office printing.
• Extreme – the smallest file. Images go down to about 96 dpi at lower quality, so photos look softer.
• Light – the best quality. Images are barely touched and the saving is smaller.
• Target size – type a limit such as 200 KB. The tool tries stronger settings step by step and stops at the best quality that fits.

There is also a tick box to turn images black and white (greyscale), which saves more on colour scans.

The saving depends on what is inside the file. A scan or a PDF full of photos usually shrinks a lot. A file that is mostly text is already small, so there is little to remove. The tool always shows the size before and after, for example 3.5 MB → 420 KB, and if it cannot make the file smaller it gives you back your original rather than a bigger file.

One more thing to know: if you ask for a very small target and recompressing the images is not enough, the tool can, as a last resort, save the pages as images. It tells you when it does this, because the text can then no longer be selected.

Step by step

1. Open Compress PDF, or open it with a size already set, such as Compress PDF to 200 KB.
2. Choose your file.
3. Pick a level, or choose Target size and type the limit from the form.
4. Press Compress PDF, read the before-and-after size, and download the file.

When the tool alone is not enough

Sometimes a document is simply too long, or the limit too small, for compression alone. These approaches reduce the content itself and use tools on this site.

1. Remove pages you do not need

A signed contract often arrives with a blank back page, a terms page you already have, or an unrelated attachment. Use Delete PDF pages to drop them, or Extract PDF pages to keep only the pages you want. Removing three image-heavy pages from a ten-page scan cuts the file by roughly a third.

2. Shrink the pictures before you build the PDF

If you are making a PDF from photographs or scans, you can keep it small from the start. JPG to PDF has a Limit file size option that compresses the photos just enough to fit a size you type. If you prefer to control each picture, Resize Image reduces their pixel dimensions and Compress Image lowers JPG quality and shows the saving. A photograph straight from a modern phone can be 4 to 8 MB. At a width of about 1600 pixels and 70 percent quality, it is often a few hundred kilobytes and still perfectly readable on a screen.

3. Turn colour scans into greyscale

A scan of a black-and-white document saved in colour carries far more data than it needs. Tick the black-and-white option in Compress PDF, or choose greyscale in your scanner app next time. For text documents the result is just as readable and noticeably smaller.

4. Split into parts

When a portal has a size limit per file and the document is long, send it in two or three parts. Split PDF can cut it by page ranges in one go, for example 1-10, 11-20, 21-30.

How small is small enough?

It helps to know the target. Many email services accept attachments of around 20 to 25 MB in total, and many government and university upload forms ask for 1 to 5 MB per file. Check the limit on the form you are filling in, then work backwards. For a form that wants under 1 MB, choose Target size and type a little less than the limit, such as 950 KB, because some portals count a kilobyte as 1,000 bytes.

Will I lose quality?

Text that is real text stays sharp at every level. What changes is the photos and scans. At Light you will hardly see a difference. At Recommended most people cannot tell on a phone or laptop screen. At Extreme, and with very small targets, photos and scanned pages look visibly softer. Judge the result at the size the reader will actually see it. If the document contains small print, a signature stamp or fine drawings, go more gently and check the output zoomed in.

Troubleshooting

The file did not get smaller. It was mostly text, or it had already been compressed well. In that case the tool gives you back your original. Remove pages you do not need, or tick the greyscale option.

The tool asks for a password. The PDF is protected. Type the password you use to open it; the compressed copy will not be locked.

The tool cannot read my file. It may be damaged. Try Repair PDF first, then compress the repaired copy.

The file is small but text looks blurry. The pages were saved as images to reach a very small target, or the scan was soft to begin with. Try a larger target or the Recommended level.

A note on privacy

Everything here runs in your browser, so the document you are shrinking is not uploaded. That is useful for files such as medical reports and bank statements, which is exactly where people tend to hesitate before using an online tool.

Worked example: a scanned application under 2 MB

A job portal accepts files up to 2 MB. A candidate's scanned application is 11 MB, ten pages, all full-page colour scans. They first delete the two blank pages with Delete PDF pages. Then they open Compress PDF, choose Target size and type 1900. The tool recompresses the scans and returns a file of about 1.5 MB, and the text is still readable at normal zoom. They keep the original 11 MB file in case the portal asks for the full-quality version.

Frequently asked questions

Keep reading

• How to Merge PDF Files Online for Free (No Upload)
• How to Convert JPG to PDF (and PNG, Photos) Online
• Guide to Splitting PDFs: Page Ranges, Batch Cuts & Avoiding Numbering Mistakes

Written by the iLoveDocuments team and checked against how the tools behave today. Spotted a mistake? Tell us and we will fix it. See our editorial standards.