I wouldn't use "pertinacious" if I were you. It's very obscure - listed in a few dictinoaries, and probably in a thesaurus (is that where you found it?), but hardly used at all.
A thesaurus lists words that are related in meaning; the closeness of the relation is variable, according to context. They are not always synonymous. "Determined' and "indefatigable", for example, can often be used in similar contexts; a person who is determined to do something may be indefatigable. But they don't have the same meaning.