You can try to convert PDF to PostScript, e.g., with pdf2ps, pdftops, …
PostScript is human-readable and can be edited with emacs.
Depending how the PDF code was created, however, the PostScript code
may be difficult to interpret, or even contain blocks of binary code.
Unless you recognize some strings like
… (some text) show …
which can eventually be modified, it is risky to make changes. As Yuri
stated, PDF (or PostScript) code is meant to be executed. Changes might
result in a buggy code, and then the interpreter or the printer will balk.