Fireballs No, Arrows Maybe
The Portable Hole states:
It unfolds into a circular sheet 6 feet in diameter.
You can use an action to unfold a portable hole and place it on or against a solid surface, whereupon the portable hole creates an extradimensional hole 10 feet deep.
This suggests that once unfolded, there is now an open entrance that is 6' wide on the outside and 10' deep on the inside.
Fireball
A hole that is within the range of a fireball would definitely put anyone/anything inside at risk because Fireball states:
The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.
The magical fireball would extend into the hole and ignite anyone/anything inside it.
There is also a potential risk that the Portable Hole could be damaged by the fire as well.
Arrows
This is more of a DM decision, because it is unclear whether or not you can see into the environs of the portable hole. I could see a case for either, but it will depend on the GM determine line of sight. If there is one, you can fire at a target, if there isn't, then you'd have to pick a location in the hole at fire at disadvantage (unseen target.)