qpdf --replace-font=F3 --remove-unreferenced-resources input.pdf output.pdf
Better yet, use pdfminer.six (Python) with a custom font override. In most cases, however, the text is not lost —it is correctly stored as character codes, but the CMap mapping to Unicode is broken. You may need to reverse-engineer the /ToUnicode CMap in the PDF. cidfont f3