Various services exist to compress PDF files online, such as I Love PDF. This poses a serious security risk for any documents that may contain personal data, and you should only use online compression sites for files that you would be happy to share publicly. Using these services involves uploading the file to a remote server, where the file could remain in place for a period of time. If you want to use an online service like this, however, you should only do so with non-sensitive documents.
If you don’t want to download additional software to compress your PDF file, you can use various online PDF compression services instead. You’ll need to open this in Preview afterwards to preview what (if any) noticeable changes in the quality have been made by the app.
Preview will immediately save an updated, compressed version of your PDF document.
