72 is Screen rez
150 is Draft Print rez
300+ is Standard Print rez
Btw, jpg pixelation can look pretty crappy with the 72/300 difference. Best to use a lossless format
EDIT: Lossless is any format that doesn't degrade to save storage. PNG & TIFF are examples of lossless formats. (There are others.) GIF is as well, but limited to 256 colors. Of those TIFF is commonly associated with print, but I prefer PNG because of the storage savings.
Either way the PDF will likely change the format inline within the PDF file itself. From that there should be options for lossless vs storage savings. (Normally set at around 80-90% - so it can end up slightly lossy anyways.)
For example, if you've got a 72x72 pic and it's jpg and you put it in a PDF for print (150+ dpi), the PDF will expand the picture to match the rest of the doc and then try to compress it again. That is why 72x looks terrible when expanded. Add to that the "artifacts" from jpg compression, and it can get very bad.
B&W is a bit more forgiving than color, but the same rules apply.