Why should anyone create PDFs that are 100's of MBs in size?
At some point "convenience" becomes annoyance. Because instead of encouraging people to do the sensible thing like breaking documents into smaller, digestible pieces (e.g. small PDFs, page images, etc.), they are encouraged to be lazy (e.g. throw a 500 page document in a scanner that outputs PDF, and upload the oversized result to one of these annoying sites). Then the average user tries to access it and she has to wait patiently while the browser tries to load this ridiculously large file, or her browser just chokes. So, "time saved" on the front end - easy scanning of docs to PDF - can often lead to "time spent" dealing with the mess on the receiving end. The uploader gets convenience, the user gets annoyance.
Why the hell would a PDF for public use be more than a hundred megabytes? You'd think the solution to people being too retarded to properly use a PDF conversion tool is teaching them how to use it, not building a website that thrashes PDFs and hogs your computer to make sure they don't annoy anyone.
And there may be some huge PDFs (think 100Mbs) that you might not want to download, just check some pages.