Would it be ok to have a pdb tag on chemistry.SE? I watch this tag on bioinformatics.SE and biology.SE (through email notifications), and now I just realized that there are many similar questions on chemistry. It's harder to monitor them without a tag. I don't have enough points to create a tag myself.
From what you are asking, it is unclear to me what the tag will be about.
For example, I recognise
pdb mainly as a data format and as such it is only one representation of a molecular-structure. If a question is about this file format it should be (probably) tagged with that, and in addition with software and probably with cheminformatics.
One could infer, that the tag is meant to be about the PDB (Protein Data Bank), but then we have the same case as we had with IUPAC, which we ruled out to have. One could then further infer, that the question is actually about proteins, but then the appropriate tag should be applied.
We do not have tags about databases (like SDBS, NIST, CSD, CCCBDB, etc.), file formats (like
xyz, etc.), chemical software (like ChemDraw, Avogadro, Gaussian, ORCA, etc.), because they tend to behave like meta tags, i.e. they are not about the chemistry itself, and a rather poor criterion to organise things.
Most of the (currently) 37 questions deal with the file format; writing it and processing it. Some only mention it as reference. Some mention it among other file formats. Therefore I also do not think that a critical mass of questions has been reached to unambiguously define the tag.