There's actually no need for any extra software to do this on OSX.
If you open a PDF in Preview (OSX's native viewer for PDF), select [file] > [export] in the menu, then choose JPEG from the dropdown. You'll now see a box to input your desired DPI. Change to 300, save and that's it.
I imagine whatever opens PDF by default on windows could do the same thing but I've not used windows in years.