I'm a new e-junkie seller. I've uploaded 10 digital products (all .pdf files) and have had several sales already. But one file refuses to download for my customers, giving them an error message saying that it is not a .pdf file (which it IS). Do you have any idea what might be causing this? I've just uploaded a re-saved .pdf file, just in case it was something in the original file, but would like to know why this happened.