The answer to this question of, why you can not attach a PDF to a PO, might be the PDF size.
To determine if the size of the PDF is the cause, locate the PDF file in your file explorer and check its size.
In most cases, when experiencing issues attaching a PDF to a PO, you may find that the size of the PDF file is larger than usual.
You can use the link below to compress your PDF, and easily resolve the issue.
Now, you can attach the compressed PDF to your PO.