[PF] If the pen is mightier than the sword...

[ArqArturo]

Would a Contract Devil with Magus levels on the Bladebound Archetype be mightier than a lowly Material Plane Pen?.

I'm constructing a more forward-approach BBEG to fight my party, and since Contract devils are a CR 10, would giving him levels (but not items, maybe some consumables, like scrolls or potions) would turn him into a CR 15?.

The blade-bound object in question is... His contracts. Essentially is a black whip made from his contracts (fluff-wise: The contracts turn black because of the power they have) that he uses to attack the players and deliver nasty spells into them.

[Yuki Akuma]

Adding class levels increases a monster's CR. Depending on whether the class plays to its strengths or not, the CR increase is either 1:1 or 2:1.

You should definitely give him extra items if you increase his level, due to the D20 System's reliance on magic items to make things challenging.

[Ailowynn]

Yup. Levels equal greater CR. I think about six levels would be the norm for him to get up to CR 15 (generally CR = level - 1). And definitely give him magic items if you want him to truly be CR 15. This (a) makes sure that he is at the appropriate power level for that CR, as much of the power level is based on wealth, and (b) gives your party more wealth when they loot the body