If you must send files as attachments, you may have limits as to file size. You may also have problems uploading large PDFs to some websites. Reducing file sizes is one way to upload large files, get attachments through your email system and save on bandwidth too. If you have a paid version of Adobe Acrobat (not Acrobat Reader), file reduction capabilities are included as part of the software. Not to worry. Lifehacker has a post with free alternatives for Windows and Mac users to reduce PDF file sizes.
Adobe has a free online file compressor that works on files 2GB or smaller, where you can select among three compression levels (low, medium, high). Mac users can use the Preview app to take advantage of the built-in compression tool. Lifehacker also has instructions for using HiPDP, but file restrictions (10MBs or 50 pages long) will drive you to the free Adobe compressor instead of using the paid version of HiPDF.
No matter what method you use, make sure you know how to reduce a PDF for those times when you run into a file size problem.