I wondered the same. ff and fi are common ligature letter pairs, maybe the original is in an encoding where those are represented with separate characters, which then correspond to § and Ö in whatever encoding we find here?

PDF is a render-only format with optional hidden metadata for semantics.

The PDF file they created knows how to paint the ligatures they used, like "fi" and "ff", but doesn't contain the `/ToUnicode` metadata for what those symbols meant.

The generated PDF happened to put the "fi" ligature at an index that is normally used "Ö", and didn't provide more information.

It was copied from a pdf, maybe LaTeX generated. All the ligatures are wrong -- not sure where the problem lies, the pdf itself, pdf reader, or the clipboard on Linux.