I am running the following code and giving me this error : FPDF error: This document (testcopy.pdf) probably uses a compression technique which is not supported by the
Looking through the answers so far, I wasn't able to resolve the issue. Here's why and you may have the same issue so this answer would be invaluable to you too.
I did not have access to acrobat, so I couldn't downgrade the version of the original PDF. I wasn't in a position to purchase the newer version of the library which has upgraded encryption.
In the end I used an online PDF converter to downgrade to version 5 (1.4)
If you are not able to find one, here's a link for completeness.
https://docupub.com/pdfconvert/
(bear in mind that the contents of a link may change)