I am using Directory Opus to administer my PDF collection by adding tags to the "keywords" field (that is, in Dopus the "Tags" field).
Since I am having issues with non-ASCII characters in third party applications, I would like to know how you encode accents and German "Umlaute" like ö, ä, ü. I believe that the PDF convention only accepts ASCII characters in that field, so no UTF-8. What solution did you use to read and write special characters?
From trying it and looking at the file:
It looks like it will use ASCII for characters that can be represented by the current codepage. So on most European systems ö, ä, ü will be saved as ASCII characters.
For characters outside the current codepage (e.g. emoji), the field is written as UTF-16 with a BOM, which works fine with both Chrome and Edge, at least. So I am not sure it's true that PDF requires ASCII for those fields, at least with modern tools, but it does seem to avoid non-ASCII if it can.