PFCRB Page 184
"Critical Hits: When you make an attack roll and get a natural 20 (the d20 shows 20), you hit regardless of your target’s Armor Class, and you have scored a “threat,” meaning the hit might be a critical hit (or “crit”)."
Does the "20" in the definition become "19-20" or "17-20" if the critical threat of your weapon is no longer the standard "20"? Does an initial roll within the critical threat range of your weapon guarantee the hit, "regardless of your target’s Armor Class"???
Nope, critical threat range is different from automatic success (or failure).
A natural 20 is an automatic success on any attack roll or save (not other rolls which might use a d20, like skill checks) and will threaten a critical hit, you must still roll to confirm.
A roll of less than 20 is not an automatic success, but can threaten a critical on an expanded crit range. However, you must still succeed on hitting the AC of the opponent to hit them at all.
As an example, a character with a +2 bonus to hit vs an enemy with an AC of 20. The character requires an 18 or better on the dice to hit. If they roll a 20 they automatically hit and need an 18 or better to confirm the critical hit. If they're using a scimitar with keen (15-20 critical threat range) they still miss if they roll anything less than 18, even though those attacks would be critical threats if they successfully met the opponents AC.
Sometimes your threat range is greater than 20. That is, you can score a threat on a lower number. In such cases, a roll of lower than 20 is not an automatic hit. For example:
19–20/×2: The weapon scores a threat on a natural roll of 19 or 20 (instead of just 20) and deals double damage on a critical hit.