Message blocked because it contains content identified as spam. AS(4810) - PDF issue

I am trying to send a PDF attachment and I keep getting the message that it's been identified as spam. I haven't had this issue until tonight, and PDF attachments worked 24 hours ago. This is happening from multiple recipients: Gmail, Yahoo, .me, etc.

Things I've tried:

Test emails without the attachment (they go through)

Test emails with a different PDF attachment that I sent yesterday (they don't go through)

Test emails with different text in the subject and message body, and with the attachment (they don't go through)

Test emails with the file as a .fdx (Final Draft) rather than PDF (they go through)

Test emails as a One Drive link rather than an attachment (they go through)

Renaming the PDF file (it doesn't go through)

So it appears to be a PDF issue. I'm accessing Outlook on my browser using Chrome on a MacBook Pro. For work purposes I need to be able to send PDF documents as attachments, so I'm hoping to get this figured out soon. I work in a television writer's room so 7 million viewers are depending on me!

Thanks so much for any advice!