A PDF is too big to email when it is full of high-resolution images. Compression shrinks those images, and most documents lose little or no visible quality.
Choose a compression level if one is offered, then compress and download the result.
Check the result before sending it: open it and confirm the text is still sharp and the images look acceptable.
On a Mac: Preview
Open the PDF in Preview.
Select File, then Export.
Set Format to PDF, then open the Quartz Filter menu and choose Reduce File Size.
Choose where to save it and select Save. Compare the new size with the original.
On Windows: Print to PDF
Open the PDF in your browser or PDF reader.
Open Print with Ctrl+P and select Microsoft Print to PDF as the printer.
Lower the quality if the reader offers a quality setting, then select Print and save the new file.
This re-save sometimes shrinks the file, but it is not a true compressor, so try the online tool first if the result is still too big.
Compression is a trade-off. Heavy compression blurs photos and fine print, so keep the original and check the compressed copy before you send it anywhere.
Common questions
How small can a PDF get? It depends on the images inside. A PDF full of photos can shrink by half or more. A text-only PDF is already small, so compression barely changes it.
Will compression make my PDF blurry? Only if you compress hard. Light or medium compression is usually invisible on screen and in print. Check the result before you send it.
Is it safe to upload a PDF to compress it? Use a tool that processes the file in your browser when you can. Avoid uploading sensitive documents, such as tax forms or medical records, to a service you do not trust.