How was it, and by who, determined that the sent '.pdf' file was received as a '.jpg' (or '.jpeg') file?
What was the name of the '.pdf' file sent?
When received, what was the name of the received attachment?
Did you send yourself an e-Mail message with an attached '.pdf' file?
If so, and when received, did you double click on the (assumed) graphic? - What application was launched which then displayed the graphic file? Also, What was the title of the graphic file's window?
This is not a bug; it is a feature. If you send PDFs Full Size, they remain PDFs. If you send them Small or Medium, they are converted to JPEG and then compressed. You will find the settings popup menu in the bottom righthand corner of the message composition window.
Thanks 'MisterMe' for mentioning the 'Image Size:' popup menu.
My quick test earlier, before my reply - this thread, consisted of a 300 KB+ '.pdf' file, and either the 'Image Size:' popup menu did not appear or I was oblivious of it.
I prefer 'Eudora' over 'Mail', and now have additional reasons why.
I created, sent, and view a dozen of differently constructed e-Mail's. Each with many '.pdf' files. The results were mixed, and thus I agree with 'adambyte'.
Some small '.pdf' files (less than 55 KB, or so) remained as '.pdf' regardless of the 'Image Size:' popup menu selection.
Multiple page and most large '.pdf' files remained as '.pdf' at 'Medium', 'Large', and 'Actual'. they did become '.jpg' at 'Small' - and only the first page was viewable.
A (possible) rule of thumb:
If you drag a file, small enough that it's graphic is displayed in the message window - select 'Actual' from the 'Image Size' popup menu (if it is not already selected), and cross your fingers as you click the 'Send' button.
It probably would not hurt to 'cc' or 'bcc' yourself the message; so you then know how your 'Mail' message was received (and viewed), as by those you sent it to.
My 1857 character, 320 word, 33 line, 37 sentences, and 17 paragraph anti-'Tiger' / MacOS X rant has been omitted here; 'adambyte's comment said it rather sedated.