This Unearthed Arcana article lists several new invocations, one of which has been slightly modified and published in Xanathar's Guide To Everything, page 57.
Improved Pact Weapon (XGtE) states:
You can use any weapon you summon with your Pact of the Blade feature as a spellcasting focus for your warlock spells. In addition, the weapon gains a +1 bonus to its attack and damage rolls, unless it is a magic weapon that already has a bonus to those rolls. Finally, the weapon you conjure can be a shortbow, longbow, light crossbow, or heavy crossbow.
"Original", UA version of Improved Pact Weapon states:
Any weapon you create using your Pact of the Blade feature is a +1 weapon. This invocation doesn’t affect a magic weapon you transformed into your pact weapon.
Superior Pact Weapon (UA only) states:
Any weapon you create using your Pact of the Blade feature is a +2 weapon. This invocation doesn’t affect a magic weapon you transformed into your pact weapon.
Ultimate Pact Weapon (UA only) states:
Any weapon you create using your Pact of the Blade feature is a +3 weapon. This invocation doesn’t affect a magic weapon you transformed into your pact weapon.
If you selected more than one of these Eldritch Invocations, such as Ultimate + Superior, would the bonuses stack resulting in a +5 pact weapon?