Could be one of several reasons. To avoid becoming a file sharing site is one. Both PDF and DOC(X) files could be carriers of nasty viruses. A large picture can be something up to a couple of MB, a sizable PDF (aircraft manual?) can easily run into hundreds of MB and server capacity becomes an issue in that case. If it's a combination of text and images and not too large, you could try copying from the document to the reply box. I haven't tried but it wouldn't surprise me if the software is clever enough to deal with a mix of text and images. If you want to share largish documents, you will need to find a bit of server space to park the document and link to that.