How do you get the hash from a PDF?

[binarystar]

Before you point me to pdf2hashcat.py, hear me out.

I tried this with a PDF file that I have, and the problem is that it returns this error:

Code:

b.pdf : Could not find /ID tag

It's a password-protected PDF that you can't read without entering the password.

The metadata I get from ExifTool is pasted below. 

Code:

ExifTool Version Number         : 10.88 File Name                       : b.pdf Directory                       : . File Size                       : 913 kB File Modification Date/Time     : 2018:03:31 21:08:54+09:00 File Access Date/Time           : 2018:03:31 21:08:58+09:00 File Creation Date/Time         : 2018:03:31 21:08:54+09:00 File Permissions                : rw-rw-rw- File Type                       : PDF File Type Extension             : pdf MIME Type                       : application/pdf PDF Version                     : 1.7 Linearized                      : No Encryption                      : Standard V4.4 (128-bit) User Access                     : Print, Modify, Copy, Annotate, Fill forms, Extract, Assemble, Print high-res Warning                         : Document is password protected (use Password option)

I can't even get through the first hurdle of getting the hash for the password. Is there any way around this?