How big is the PDF? There are size limits on email and text messages, if we're talking about either one of those. Be sure your attachment is not exceeding the limit.
Did this PDF come to you via your inbox? Via the same messaging type you are trying to send it?
Again assuming email, have you tried sending it from a PC or laptop? Transfer it via USB to your PC and send it as an attachment from there. If it doesn't go through your PC will probably tell you why it failed. Depending on the app and the error mail apps on devices don't always give you all of the information.