[Wu Jen]

If you cast Improved Invisibility on a target that did not want it cast on them can they resist it? Same with Silence or any other spell that does not give a resistance to have it be cast on them.

I.e. Casting Silence on a guard then sniping guard so that the body hitting the floor doesn't make any noise and alert other guards.

Casting Improved Invis on a bound guard that you gagged and put in a closet. Etc.

Thanks!

[Michael Chandra]

No, there's no resistance described to prevent you from being 'buffed' by such a spell. A GM could of course always add a resistance roll for unwilling targets. But then again, that means you wouldn't be able to use these for really fun tricks.

[Stainless Steel Devil Rat]

Indeed.  Spells like Armor and Heal do not spell out resistance tests because it's generally assumed you wouldn't want to resist them.  But if you're trying to make a hostile target glow via Armor, or ruin an enemy's chance for a good heal for his wounds by casting a weak-ass heal on him, it's a good rule of thumb to have a principle that no spell is unresistable and that there's always a chance to resist.

[Xenon]

Casting Silence on a guard then sniping guard so that the body hitting the floor doesn't make any noise and alert other guards.

Silence is an area of effect spell (you target an area, not a specific guard)
Any nearby listeners get to oppose the illusion spell.

Casting Improved Invis on a bound guard that you gagged and put in a closet. Etc.

Any potential observer get to oppose the illusion spell.