Thanks, I already have ghostscript, did some more searching and found;
https://askubuntu.com/questions/872482/adding-a-logo-in-a-pdf-by-ghostscript My infile test.pdf contains only one line with the word "test.pdf"
(No need to, but I changed "Sample" to "AddedText" and saved it as sample.ps)
Running
gsos2 -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=out.pdf sample.ps -f test.pdf
and it produced the attached out.pdf
So, it looks like this can be done without conversion.
Just noticed that it also added the number "1" in the lower right corner. Guess this can be controlled
by the ps-file, font, size, where to place text etc.