Mexico entry rules vary by nationality, but most leisure travelers arrive with a passport and are processed under a visitor permit system (often referred to as the FMM or the digital FMMd for some air arrivals). Your allowed length of stay is granted by immigration at entry and can be less than the maximum, so be prepared to show return flight and lodging details if asked.
If you enter Mexico by air, you may be directed to download a digital record after arrival. If you enter by land, you may need to complete an FMM process in advance or at entry points.
Puerto Vallarta (Jalisco) and the Nayarit side of the bay do not use the Quintana Roo VISITAX system, which applies to Cancun and the Riviera Maya region.