We systematically dope monolayer graphene with different concentrations of nitric acid over a range of temperatures, and analyze the variation of sheet resistance after vacuum annealing up to 300 °C. The optimized HNO₃ doping conditions yield sheet resistances as low as 180 Ω sq.⁻¹, which is significantly more stable under vacuum annealing than previously reported values. Raman and photoemission spectroscopy suggest that this stable graphene doping occurs by a bi-modal mechanism. Under mild conditions the dopants are weakly bonded to graphene, but at high acid temperatures and concentrations, the doping is higher and more stable upon post-doping annealing, without causing significant lattice damage. This work shows that large, stable hole concentrations can be induced by transfer doping in graphene.