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 in your file explorer the PDF which you would like to attach to a PO, and check out the size of it.
In most cases, when having an issue attaching a PDF to a PO, you will see that this PDF seems to be larger than any PDF.
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.