Convert PDF to MID

Several utilities, such as PDFtoMusic, will let you import Adobe Portable Document Format Files containing sheet music to then export those to various formats, including MIDI Music Data Files (.pdf to .mid conversion). 

Such attempts will only be successful if your PDF documents actually contain music data, typically exported from music notation and composition programs.

A viable alternative would be using an optical music recognition (OMR) solution, such as SharpEye Music Reader, which would be capable of recovering sheet music from images or PDF documents.