quick question??

is rainwater bad for my plants.......

That's a silly question.

it might be a silly question to you, but i was thinking the ph of the rain water may be to high....

So clarify your question. What you are asking is if bringing rain water in for indoor plants right?
I would think that would be fine except like you have stated the ph might be off. So just test ph.
I mean no dis respect on your question.