Why does the file have to be under 5mb?
If it is just for emailing a proof, you can open the pdf in Acrobat & then File/Reduce size.
If it is to send to a printer, we are a large format printing house & have an ftp site where all our customers can download their files - size is not an issue. This way they can download large pdfs, photoshop files or illustrator files (along with the support files} & do not lose any quality.
If this is the case, ask your printer if you can download it instead of emailing it. Stuff or zip the files before downloading so nothing gets scrambled in the transmission.