When a maker makes a PDF, he can set the maximum print resolution to 150dpi if he chooses, and lock it in to the file, so you can get really crystal clear PDFs that are limited to print at low resolutions.
The PDF is best suited to
DMing and/or reading from a laptop.
DMing from digital is tricky and takes a lot of getting used to, but there seem to be a lot of people who have gone down that road. Reading from PDF on screen for long periods of time is a pain, but for reference product that you just dip into from time to time, and for the sheer convenience of being able to carry your entire D&D library on a laptop, PDFs are just what you need.
Don't splash too much cash. At the end of the day, it's not a real product that's gonna maintain or appreciate in value. It's a dead investment, so think of PDFs the way you think of buying a newspaper or groceries. They fill a need, but do not ever give a return.
This week I've been mostly eating . . . The white ones with the little red flecks in them.