My dad has a pacemaker. He seems to still need to take nitroglycerin tablets for his heart, though not as often as before the pacemaker was implanted. Is it safe to take the tablets?

Fundamentally, no. The doctor should be consulted. Here is the prescribing information: http://www.pfizer.ca/english/our%20products/prescription%20pharmaceuticals/default.asp?s=1&id=82&doc=enmonograph

It's possible that the pacemaker can be adjusted wirelessly external to the body.

If the pacemaker is not doing enough, the doctor should be seen "at once" and consulted on the problems he is having