I don't own a mac (wish I did), but I'm wondering if the pdf's that open automatically were created on a Mac, while the others were created using some other software? Just a shot in the dark..
Why do some pdfs, that are attached to an email, open when the email is opened while others require clicking on the attachment icon to open?
Not a mac person here. It may have more to do with how the sender's email client deals with attachments than it does with the attachment or your computer being a mac.