How to compress a PDF on a Mac

1. Why some PDFs are so big

Text barely takes any room. Pictures do. A scanned document is a photo of every page, and a brochure or a statement with a logo, photos or charts carries every one of those images at full size. That's what compressing shrinks: the pictures inside. A PDF that's nearly all text is already small, and no tool will shrink it much.

2. How to compress a PDF on a Mac with Preview

1. Open the PDF in Preview. Double-clicking it usually does, or Control-click it and choose Open With > Preview.
2. Choose File > Export.
3. Leave Format set to PDF.
4. Open the Quartz Filter menu and choose Reduce File Size.
5. Give it a new name, so you keep the original, and click Save.

Check the result before you send it. Reduce File Size is aggressive: it brings every picture down a long way, which is fine for text with a logo, but can leave scans and photos soft or blocky.

3. Better quality: make your own filter

macOS lets you make a gentler version of Reduce File Size, and Preview lists it alongside the built-in one.

1. Open ColorSync Utility. It's in Applications > Utilities, or press Command-Space and type ColorSync.
2. Click Filters in the toolbar.
3. Find Reduce File Size, click the arrow at the right of its row, and choose Duplicate Filter.
4. Open the copy. Under Image Sampling, set a higher resolution, such as 150 pixels per inch. Under Image Compression, move the quality slider towards the better end.
5. Give the copy a name you'll recognise, like Smaller PDF, Good Quality.

Next time you export from Preview, your filter is in the Quartz Filter menu. A higher resolution keeps scans readable; a lower one makes smaller files.

4. Get under an email or upload limit

Gmail takes attachments up to 25 MB and Outlook.com up to 20 MB, and many upload forms stop at 5 or 10 MB. Preview can't aim for a size, so work up to it:

1. Export with your gentle filter first, then select the new file and press Command-I to see its size.
2. Still too big? Export the original again with Reduce File Size.
3. Still too big? Split it. In Preview, choose View > Thumbnails, select some pages and drag them to the Desktop to make a separate PDF, then send it in two parts.

Always start from the original, not an already-compressed copy. Compressing twice loses quality twice.

5. A word on online compressors

Search for this and most results are websites that compress a PDF for you. They work, but you're uploading the document to someone else's server to do it. For a bank statement, a contract, a payslip or anything with an ID number on it, use Preview, or an app that does the work on your Mac.

6. Shrink PDFs as they arrive

If you're compressing the same kinds of files every week, like scans, statements and invoices, do it as they land instead. iFiley is a file catcher for Mac: when a PDF or an image downloads, it asks which folder it goes in, and can shrink it on the way. Pick a size in Settings, under 1, 2, 5 or 10 MB, and it uses the best quality that fits, or the smallest it can make if nothing does. Only the pictures inside get smaller, so the text still searches and selects, and a file that's already small enough is filed as it is. It all happens on your Mac.