PDFs are platform independent, the company makes a free reader, and it's displayed more constantly than PPTs and DOCs on different platforms/versions.
I must say though that I dislike Acrobat Reader (Adobe Reader, call it what you want) 6 and 7 and I downgraded to the relative simplicity of version 5.
Some profs on my uni have problems with files but I guess that's not limited to PDF. Once I accidentially printed a 70 pages PDF from the slides of one lesson. Ah well, not entirely, the hi-res background picture slowed the printing process down and I noticed something isn't right

Others plaster their presentations with pictures.
On the other hand, I always chuckle if I see students who print six or even nine slides on one page and then try to learn with this stuff. To each his own I guess.
