Alright, I'm feeling very silly for having to ask this, but how do I upload .pdf files? I created a tutorial in PDF format, but when I tried to upload it got a message that my 'PNG image had the wrong file etension'? I tried uploading other .pdf files, but got the same message, so basically I can't upload pdf files. Does anyone what I'm doing wrong, or how to fix this?

Thanks in advance!

EDIT: I CAN upload other file formats such as .jpg and .png etc.