Can the Rod of the Pact Keeper +1 (DMG p. 197) be chosen as a pact weapon by a Pact of the Blade warlock?
The Rod is an arcane focus, not a weapon. It could be used as an improvised weapon, but it is still not a weapon.
Rules as fun
It sure seems similar to a club or quarterstaff to me. A DM certainly could choose to accept it as a +1 version of either of those weapons.
Per the 3rd paragraph for Pact of the Blade:
You can transform one magic weapon into your pact weapon...
Previously in the Pact of the Blade description, it states that your pact weapon is a melee weapon, so presumably you can only do this with a melee weapon, though, RAW this is actually a bit vague so you might have some basis to make an Oathbow your pact weapon.
Regardless, the Rod of the Pact Keeper is strictly an arcane focus, per the description for magic rods (DMG, p. 139) and the list of arcane foci (PHB, p. 150).
This is not be confused with magic staves, which can all be assumed to be quarterstaffs and thus would be a viable candidate for being a Pact Weapon.