This typically happens if it was uploaded incorrectly or your system messed up with the download, probably the former. Does this occur with all PDFs or just a few? Do have a link so we can try it? What OS and version of Acrobat are you using?
As for the upload issue, folks sometimes upload a PDF as ASCII and it is a binary file. The ASCII upload corrupts the file. The first need is to see if it is an upload issue or something else, so the URL would be helpful.