I am trying to split a pdf into its pages. For few documents I get the error: PdfReadError: File has not been decrypted

I tried the following fixes, but nothing works.
Fix 1: Manually decrypt the file using: inputpdf.decrypt('')
This throws an NotImplementedError:

Fix 2: Suggested here: https://stackoverflow.com/a/48364988/15368105
This also doesn't work and throws the same error.

Is there a solution for this?