There ist a big difference!
PBS = Phosphate Buffered Saline, meaning (physiological) salt in a phosphate buffer, pH7,4. PBS is more or less defined, you will find similar protocols for preparation.
PB = phosphate buffer, without salt. Phosphate buffers are prepared by sodium or potassium ions of mono- and dihydrogen-phosphates, the pH depends on the relationship between the mono- and the dihydrogenphosphate concentrations, a good overview about the necessary relationship give the Lab FAQs from Roche (just google) and you can prepare a 1mM phosphate buffer as well as an 50mM phosphate buffer or other concentrations. So concerning "PB" you need more information (concentration and pH). In that protocol you are citing they probably mean pH7,4.
Note also that there are more people not knowing the difference, so in many labs people use the some word for phosphate buffer, PBS and sometimes also for PBS-Tween or PBS-Triton (both often abbreviated with "PBST") used for washing in immunoassays. So if someone tells you to wash your western blot with "phosphate buffer" better ask if the really mean "phosphate buffer" and not PBS-Tween.