Can I put holes in every floor joist?
Drilling a hole in a 2x8 joist would not be a problem, the building codes have guidelines and you could drill holes big enough for 1/2&quot; conduit within those guidelines.  However, your roof is not regular joist and rafter construction, you have trusses which are engineered and manufactured and there are no general rules for drilling and notching them.  You'd have to consult the manufacturer or engineer.  (In fact I think you'd have to check about whether it's suitable for storage, those mending plates in the bottom chord make me think not - but that's a different question.)
Even if it was joists - did you think about how you'd thread sections of EMT through the holes you drilled? It's pretty tricky, it has some flex and you can oversize the holes some to gain some wiggle room, but still.  You  have 10' sticks of EMT and trusses 2' apart on centers ... think about that a minute.  Believe me, you don't want to do that, it would be the hardest possible way to do the job.
There are a number of better alternatives for routing the EMT.  You could run it on top of the bottom chord of the trusses, or hang it from the vertical or angled members.  I think my preference would be to mount the boxes on the bottom face of the angled members of the trusses, that would give you easy access to the wiring inside and with hangers, it would be easy to support and little to no conduit bending.