For about a month now emails I send with PDFs attached have not been received. The only remedy that works is to attach to a reply from my intended recipient.

Nothing has changed my end. Perhaps a Thunderbird update is causing the problem? Emails I send with attachments to Distribution Lists seem to be getting through. I've tested what is happening by sending an email without attachment to a particular recipient, followed by an email with an attachment. The recipient reports only the email without an attachment is received. I then sent the attachment via a Reply email and it was finally received!