How to Compress a PDF Without Losing Quality

A PDF that won’t attach to an email is one of those small, recurring annoyances. The fix is compression — but “compress” gets thrown around as if it’s one button that always works. It isn’t. Knowing what compression changes is the difference between a crisp 2 MB file and a blurry mess. I’ve squeezed thousands of PDFs down for email and uploads, and here’s how I do it without wrecking quality.

What “compressing a PDF” actually changes

The single most useful thing to understand: most of a PDF’s size is images, not text. Text and vector graphics are tiny. A scanned page or an embedded photo is huge. So PDF compression is really image compression — it downsamples pictures to a lower resolution and re-encodes them more efficiently.

That tells you two things:

• A text-only PDF (an exported document, a report) is already small. Compressing it barely helps, and that’s fine.
• An image-heavy PDF (a scan, a portfolio, a slide deck) is where compression pays off — sometimes shrinking the file by 80–90%.

Method 1: On a Mac (Preview, offline)

macOS can compress PDFs without any download:

1. Open the PDF in Preview.
2. Choose File → Export.
3. In the Quartz Filter dropdown, pick Reduce File Size.
4. Save.

The catch: Apple’s built-in “Reduce File Size” filter is aggressive and can make images too soft. If the result looks rough, the better move is a custom Quartz filter with a gentler resolution (around 150 DPI). You create one in the Mac’s ColorSync Utility, and it then appears in that same Export dropdown. It’s a five-minute, one-time setup that gives you reliable, offline compression for good.

Method 2: On any platform (Ghostscript, offline + scriptable)

If you want repeatable, offline compression with real control, Ghostscript is the tool the pros quietly use:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.5 \
   -dPDFSETTINGS=/ebook \
   -dNOPAUSE -dBATCH -dQUIET \
   -sOutputFile=compressed.pdf input.pdf

The key dial is -dPDFSETTINGS:

/ebook is my default — it’s the best balance of size and clarity for anything that’ll be read on a screen.

Method 3: Online (fast, any device)

When I’m not at my own machine, a web compressor is the quickest route: open it, drop the PDF in, pick a compression level, download. It works the same on phones and Chromebooks. Online tools differ in how they handle your file, which is worth knowing before you pick one.

When compression won’t help

• The PDF is already mostly text — there’s almost nothing to compress.
• The images are already low-resolution — squeezing them further just adds blur.
• You need print quality — don’t drop below 300 DPI.

In those cases, if the file is still too big to email, don’t fight it: upload it to cloud storage and share a link instead. (That’s also the right answer for very large files in general — see our guide on reducing PDF size for email.)

My quick rule of thumb

For email attachments, try /ebook in Ghostscript or a 150 DPI custom filter in Preview. Both work offline. The reduction depends on the original PDF; inspect small text and images before sending the compressed copy.