Informing specialists about a new manuscript posted on the arXiv is a common practice. Once it is posted there is no need to send a file.

But on my opinion, asking explicitly a person you do not know to read and comment is not polite. You can write something like this: "I would like to bring to your attention the new preprint... I would appreciate any comments".

Informing specialists about a new manuscript posted on the arXiv is a common practice. Once it is posted there is no need to send a file.

But on my opinion, asking explicitly a person you do not know to read and comment is too intrusive and impolite. You can write something like this: "I would like to bring to your attention the new preprint... I would appreciate any comments". Or "I will be grateful for any comments".