will it keep?

Should, might not taste as good but you can eat it

As long as it was refrigerated properly and kept cold it should OK on Friday. Most of the safety rules you see published say to use any leftovers within 1-2 days, but they have to recommend that time frame in order to avoid being sued. I normally use leftovers within 1 week, but have had some much longer than that which were OK. I've also had some go bad more quickly.

Always visually inspect your leftovers and give them the smell test. If you smell an off odor, don't eat it.

Yeah, it'll keep. Transfer it to a container, don't leave it in the can, or it'll taste like metal after a day. 

Yeah, I second the above, it might not taste very good, surely not as good as on day #1.  I assume you want to do this for convenience.  If it were me, I might dish up the pasta salad by itself and portion out my tuna into little sandwich/snack plastic baggies, seal it, and just lay that on top of your salad in the bowl and put it in the fridge.  You can just dump it into your salad and mix it up and eat it.  It might help maintain the taste and quality of your salad over the several days.  Tuna sometimes does weird things when mixed in with other things over the course of a few days.