I have trouble to sending e-mails attaching a pdf file. What is wrong?

Are there anybody here who have trouble to attaching a pdf file and sending with an email?
I have had problems now for approx. 4 days. If I send an e-mail without a pdf, it will reach the recipient. But if I attach a pdf, the e-mail come back to me with a message that:
This email has been rejected. The email message was detected as spam.
And I have tried to send with 6 different g-mail accounts, and also from Protonmail account, - and all works when I send without a pdf file as attached, - but as soon as I attach a pdf they end up in spam ... or I get this message back.

These pdf files are invoices which are saved as pdf.

I have been sending these pdf files for many years with no problems. So this has been happening right now lately.
I would be happy if I got a feedback from Adobe, if there is something wrong with PDF files at the moment?

Thank you very much. I look forward to hearing from you soon