The 300-ppi thing is mostly just easy to remember and provides some wiggle room for sizing and cropping the photo once it’s placed into a layout application.
Even there, the 300-ppi rule of thumb only makes sense, as @rgsjoel mentioned, in conjunction with ≈150-line halftone screens (or their digital or stochastic equivalents). And even that only makes sense when the printed materials are intended to be viewed at arm’s length. When viewed from further away, the resolution can decrease accordingly.
Of course, all that’s too hard to explain to people, so 300ppi ends up as a rule of thumb that few people understand or, worse still, regard as some kind of official printing requirement.
With digital printing, the same amount of data is required to get similar results. It’s not as though digital printing is able to extract more image detail out of the image file than exists within the file.
I typically use the same resolutions for digital as I would do if I were preparing the same file for offset.
Those are the worst things to do in the world - no PDF compliance and can destroy valuable data within the background of the document that is essential.
Acrobat Professional is the only tool to reduce PDF sizes accurately and maintaining all essential data.
seconding Smurf2
Any PDF sent out for print should conform to the printer’s PDF specifications and, where available, using the printer’s Job Options. Running a PDF through any kind of compression software destroys the printability of said PDF. Full stop.