You can find programs that can process a PDF with music sheets or other audio elements and export or save them to something else, typically a music sheet (.mxl), or in some cases directly to MIDI format and that's what most users have in mind when searching for pdf to midi conversion.
Programs like the PDFtoMusic allows users to convert pdf to midi. This only works with .pdf documents that contain music data, usually exports from music writing programs.
A viable alternative to this would be to use some Musical OCR (OMR) solutions that can processes scanned music sheets in .pdf files and generate a sheet / music score from it which can then be turned to a .midi file.
Updated: March 31, 2017