Thank you for your post.
I am using FileMaker Pro 10 on Mac OS X 10.5.6. I frequently attach a file (usually PDF) using the Send Mail step (email client) and Apple Mail sends the message successfully each time. I have not encountered a crash. I'm well aware that doesn't help you, so let me try to find out more information?
Do you ever crash when you don't include an attachment?
Are you creating the 1 page PDF from FileMaker Pro? If so, is it via "Save/Send Records As -> PDF"? (I have seen FileMaker crash during this step).
Do you notice if a specific amount of records are sent successfully before the crash?
Are you running OS X 10.5 or 10.4?
Any additional information you can provide may help me narrow down the possibilities.