How to approach experts to get feedback on a paper
Generally it's considered okay to e-mail researchers who you don't know and share your work with them. This is especially true if you are a student.

Be conservative about it -- pick only a few people whose research interests align closely with the paper you're writing. (For example, whose work are you citing?) Don't e-mail famous people just because they're famous.

That said, I think that asking for detailed feedback might be seen as presumptuous. Rather, just tell them that you would like to share your work with them, describe it very briefly, and say something like "If you have any comments I would be grateful to hear them."

Probably most people will ignore your message. (They have no obligation whatsoever to read your message or respond.) Maybe one or two will write back with general feedback or suggestions, and if you are very lucky you might get some detailed feedback. What you might realistically hope for is a response along the lines of "Thanks, this looks interesting, you might be interested in X, Y, and Z" which at least gives you some related works to check out.