How to Compress a PDF
Shrink a PDF that is too large to email using Preview on Mac, Word's minimum size option, Microsoft Print to PDF, or Adobe's free online compressor.
A PDF is too big to attach or upload. Images are almost always the reason, so every method below works by re-encoding them at a lower resolution.
Export with the Reduce File Size filter in Preview on Mac
Open the PDF in Preview and choose File, Export. Leave Format set to PDF, open the Quartz Filter dropdown, choose Reduce File Size, and click Save. Give it a new name so the original stays untouched. This filter cuts size dramatically but also drops image quality a lot, so open the result and check that text in scans is still readable.
Save from Word with Minimum size
If the PDF came from a Word document, go back to the document. On Windows, choose File, Save As, set Save as type to PDF, choose Minimum size (publishing online) under Optimize for, and click Save. On Mac, choose File, Save As, pick PDF, and select Best for electronic distribution and accessibility.
Reprint to PDF on Windows
Windows 11 has no compress command, but re-saving through the print system often helps. Open the PDF in Edge, press Ctrl + P, choose Microsoft Print to PDF as the printer, and click Print. Compare the new file size with the old one. This works best on PDFs with large embedded images and does nothing for files that are already lean.
Use Adobe Acrobat online
Go to adobe.com and open the free Compress PDF tool. Upload the file, pick a compression level, and download the result. The medium setting usually halves a scanned document with no visible change. This is the only practical option on iPhone, which has no built-in compressor.
Check the result
Open the compressed file and zoom in on a page with small text or a photo. If it looks rough, go back and use a gentler setting or a different method.
Better control on Mac. The built-in filter is aggressive. Open ColorSync Utility from Applications, Utilities, click Filters, duplicate Reduce File Size, and raise the image resolution to 150 pixels per inch and quality to medium. The copy then appears in Preview's Quartz Filter list.
Scans compress the most. A page scanned at 300 dpi in color is often 5 MB or more. Rescanning at 150 dpi in grayscale, or compressing the existing file, brings it under 1 MB. Web tools upload your file to their servers, so avoid them for sensitive documents.