552 Spam Message Rejected

[#1]

Email being sent to one of my accounts being rejected as Spam (it isn't). It's a PDF attachment. Any ideas how I can get this unblocked and prevent recurrence? It is not the sender that is the problem as they sent the email to an alternative address I provided, when I forwarded it to the PN mailbox I got the same error the original sender did. Have posted the error reports as additional info. I suspect the problem is in the PDF somewhere but what like to know what it is so I can advise the sender if it is something they need to fix their end.
Thanks
Andrew

[#2]

Get rid of that file until you remove your email address etc. from it

[#3]

Oops - in a rush - I meant to redact that - thanks for spotting - will re-post shortly with an appropriate edit
Andy
p.s. hopefully that's better

[#4]

Probably something to do with the reputation of the sender's domain rather than the content of the email itself. I notice this post is quite old, is it still happening?