I just purchased Book Publishing DIY by Tony Loton.
Tony discusses acronyms and provides a list of some of them. Unfortunately, most of what he considers to be acronyms are abbreviations, such as ISBN, PBL and SSN.
An acronym is an abbreviation that can be pronounced like a word, such as "radar," "scuba" and "NAFTA," and probably has one or more vowels.
For "PBL" to be an acronym, it could be pronounced "pebble" or "peeble" -- if it received some vowels.
The obscure term (meaning "Published By Lulu") is much more likely to be pronounced "pee bee ell," and is therefore NOT an acronym.