How to Reduce PDF File Size - Offline, Online, and on a Mac

Step by step

1. 1

   Check what weighs the most

   A large PDF almost always means photos or scans inside, because the text alone weighs next to nothing. If the document runs to several or a dozen megabytes, high-resolution images are to blame. They decide how much size you can actually shave off.

2. 2

   On a Mac, use Preview and a Quartz filter

   Open the file in Preview, go to File → Export, and pick Reduce File Size from the Quartz Filter list. Save under a new name so you don't overwrite the original. This filter can seriously degrade photo quality though, so if the print becomes unreadable, go back to the original.

3. 3

   Build your own Quartz filter for better quality

   The built-in filter can be too aggressive and ruin scans. In the ColorSync Utility app, you can create your own filter and set the image resolution to about 150 dpi with medium JPEG quality. That filter then shows up in Preview's list and gives you a compromise between size and readability.

4. 4

   Export from Word at minimum size

   If you have the source document, open it in Word and choose Save As with PDF as the file type. Click Options or select Optimize for: Minimum size. Word embeds the photos at a lower resolution and the file comes out noticeably lighter. The free LibreOffice has a similar option when exporting to PDF.

5. 5

   Go online when you have neither a Mac nor Word

   Tools like iLovePDF or Smallpdf compress a PDF in the browser within seconds and are free for single files. Pick medium compression so you don't wreck the quality. Never upload contracts, ID scans, or documents with personal or medical data to them.

6. 6

   Scan smarter next time

   If the PDF is a scan, the weight comes from the scanning resolution. For ordinary text documents, 200-300 dpi in black and white or grayscale is plenty. A color scan at 600 dpi can weigh a dozen times more than you really need. If your scanner saves straight to PDF, set a lower resolution and black-and-white mode before you even scan.

Free and offline vs online tools

You can shrink a PDF for free in several ways without sending it anywhere. On a Mac you have Preview, and on any computer with Word or the free LibreOffice you have PDF export with size control. Windows itself lacks a good compressor: the Print to PDF feature sometimes helps a little, but it doesn't shrink the images inside, so results vary. The paid Adobe Acrobat Pro, in turn, has a ready-made Reduce File Size feature and a PDF Optimizer that shows what weighs the most in your document.

Online tools are convenient and often compress harder, but you're uploading the document's entire contents to someone else's server. With confidential files, that's a real risk, because the data goes to an outside company, sometimes in another country. For an invoice to a friend, online is fine; for documents with a national ID or bank account number, stick to the offline methods.

Upload limits on government portals and forms

Government systems, from e-filing portals to official inboxes and tax forms, put size limits on attachments. They usually sit in the range of a few megabytes per file, but the exact value depends on the given system, so check it in the form before you start uploading. If the file is too big, you'll get an error the moment you attach it.

If the document still won't fit under the limit, split it into parts or rescan it at a lower resolution and in grayscale. With multi-page scans, that's usually more effective than compressing the finished file. Text sharpness at 200-300 dpi is perfectly sufficient for a clerk to read it.

Frequently asked questions