How to compress a PDF without losing quality.

Why the file is so big in the first place

A PDF is a container. The text in it is tiny — a hundred pages of plain type is often under a megabyte. What makes a PDF heavy is nearly always one of three things:

• Scans. A scanner produces one full-page image per page, frequently at 300 or 600 DPI, and stores it losslessly. Twenty scanned pages can easily be 40 MB.
• Phone photos dropped into a document. A modern phone photo is 12 megapixels. Placed into a report at postcard size, it still carries all 12 million pixels — the page just draws it smaller.
• Export settings nobody changed. "Print quality" or "press quality" presets in Word, Pages, InDesign and Canva embed images at far higher resolution than a screen or an office printer can show.

That is the useful insight: compressing a PDF is really compressing the pictures inside it. Which is also why the results vary so wildly from one file to the next.

What compression actually removes

File Guru's Compress PDF tool works by re-rendering every page at a chosen resolution and re-encoding it as an optimized JPEG. That is genuinely effective — an image-heavy or scanned PDF often drops by 70–90% — and it comes with one trade-off worth knowing before you click:

For the overwhelmingly common case — a scan, a signed contract, a photo-heavy portfolio you need to email — that's a fair trade. For a text document you'll need to search or edit later, it isn't, and the section further down has better options.

Step by step

1. Open the tool. Go to Compress PDF. Nothing is uploaded anywhere — the file is read and re-rendered inside your own browser tab, which is also why it works offline once the page has loaded.
2. Add your PDFs. Drag one file on, or select several. A batch is compressed with the same settings, which is what you want when you're shrinking a folder of scans.
3. Pick a compression level. Three presets: Best quality, Balanced, and High compression. Start at Balanced.
4. Press Compress PDFs and read the before-and-after sizes shown for each file.
5. Download the files one at a time, or all of them together as a .zip.

Choosing between the three levels

They differ in two things at once — the resolution each page is rendered at, and how hard the JPEG encoder is pushed:

• Best quality renders pages above their nominal size before encoding, so fine print and thin lines survive. Use it for anything that will be printed, or a scan with small handwriting.
• Balanced renders at roughly the page's own size. This is the right default for reading on screen and for email, and it's where most files land at a sensible size.
• High compression renders below page size and encodes aggressively. Use it when a hard upload limit is the only thing that matters and legibility of body text is enough.

Two extras sit under the advanced options and are easy to miss:

• Target size, in MB per file. If the real requirement is "under 5 MB", set that instead of guessing at levels. Each PDF is then tried at several quality levels and the best-looking one that fits is kept. It takes longer, because a file may be re-rendered up to three times, and it overrides the level you picked above.
• Convert pages to grayscale. Dropping colour from a document that's mostly black type on white — a contract, a bank statement, a form — cuts size again with no visible loss, because the colour was never carrying information.

Three times compression won't help

Knowing when to stop saves more time than any setting:

• The PDF is already text-only. There are no oversized images to shrink, so there's little to win — and you'd be trading away selectable text for a few percent. Leave it alone.
• It's already been compressed once. Re-encoding an already-lossy JPEG mostly adds artefacts rather than removing bytes. If a second pass barely moves the number, that's your answer.
• The problem is page count, not page weight. A 300-page document that needs to go out as three chapters isn't a compression problem. Use Split PDF to pull out a range, or Organize Pages to delete what nobody needs to read.

And if you need the text to stay searchable, don't compress at all — go back to whatever produced the PDF and re-export it at "screen" or "smallest file size" quality. That shrinks the embedded images while keeping the text as text, which no re-rendering pass can do.

Is it safe to compress a PDF online?

It depends entirely on where the compression happens. Most online PDF compressors upload your file to a server, process it there, and hand back a download link — which means a contract, a medical record or a passport scan has left your device and is sitting on someone else's disk under a retention policy you didn't read.

File Guru doesn't work that way. The tool loads the PDF with your browser's own file API, renders and re-encodes the pages locally, and hands you a download generated on your machine. There is no upload step, no server copy, and no account required. You can confirm it for yourself: open the tool, disconnect from the internet, and compress a file — it still works.