This is Add PDFDocEncoding to standard codecs, right? I think you’d need to wait for some feedback before going as far as writing a PEP. Have a look at PEP 1 – PEP Purpose and Guidelines | peps.python.org.
People might even tell you that this is not worth a PEP and should just be done in an issue.
Additionally, if it’s a PEP you will need a core developer to sponsor your PEP, as explained in PEP 1 – PEP Purpose and Guidelines | peps.python.org.
I’m just a lurker here, but that’s my understanding of how it works.