PDF upload forms often restrict the size of the submitted document. For example, the Cambridge Part III dissertation must be under 20 MB, and there are application forms where the “examples of work” submission is limited to a few megabytes. What if yours is slightly bigger than the limit?
If the document contains any high-resolution images, you can shrink them, but this is of limited use – at some point, you’ll end up with blurry images. In my experience, at this point fiddling with image compression settings won’t help you, since pdflatex will recompress everything anyway and your hard work on PNG optimisation will go to waste.
If you just need to trim the file by 10-20%, the solution is surprisingly simple. Ghostscript’s compression is slightly better than what pdflatex accomplishes, so you can just do something like the following:
ps2pdf document.pdf document-compressed.pdf