Emailed PDFs automatically opening

I've got a problem where, if I email PDFs to my gmail account and open them in mail on my iPad, it's automatically opening some of them (not all) in the message body. This is fantastically annoying, as I then don't get the option to open them in a separate window and send to Dropbox/Evernote/etc. The only 'reason' I can see why this is happening with some but not all is that it's size-related (my latest two troublesome PDFs are both under 130kb). Can't find any settings that will let me fix this. I presume I'm missing something simple so if someone is able to enlighten me, I'll be really grateful!