You can find many PDF scores on the Internet.
capella-scan 8 opens and recognizes PDF files.
In versions 5, 6 and 7 you need the free software "Ghostscript" to do so. You will find this software here:
PDF scores on the Internet may have been created in two different ways: Some of these files are scores which were written with an up-to-date notation software and then exported to PDF. Such files will be recognized almost perfectly by capella-scan.
Other scores, however, were scanned from sheet music and then saved as PDF. These are often old editions, copied, yellowed, worn, notes scribbled all over them... The performance of capella-scan might be limited by the quality of the original.