It's an old abbreviation in the chemical world.

For HN audience, the most recognizable older source that uses it a lot might be Neal Stephenson's Zodiac from 1988 :), but the chemicals are much older than that.

It is well known in the high voltage world, many old (it was banned around 1985) transformers and capacitors were filled with it.

Yep, and the disposal method for those old transformers was far too often "throw it in the woods down the street".

Every so often the local news will do a story when someone finds another pile of rusty and leaking transformers sitting on a lot behind a daycare or something.

Poly-Chlorinated Biphenyls? What other acronym would they use?

Or is it just that some might find it contentious with Printed Circuit Board?

Three letter acronyms are always suffering from name space collisions, in mixed or broad context.

It’s really a symptom of using three letter acronyms at all, rather than any one acronym being reasonably preferable to another under any circumstances. When you only have a symbolic trigram combining any of 26 letters, reuse is not just inevitable but frequent.