Replace pdf.ttf with sharp2.ttf, keep name the same

As discussed at length in issue tesseract-ocr#182, the existing pdf.ttf causes difficulties
for certain PDF viewers, in part because the old file had zero advance width.

With testing, sharp2.ttf seems to be the best available compromise, although
it's not perfect and causes some visual difficulties in Evince.  It does
seem to fix Kindle and OS X Preview.