Is it appropriate to ask for an opinion about a preprint from researchers I don't personally know before submission?
In my field (mathematics) it is somewhat common to send out preprints like this, especially for junior researchers like yourself.
In my experience, the most common way is to send a copy of the paper as an attachment, briefly introduce yourself and the paper, and say something like &quot;I would welcome any comments if you have any. Thank you very much.&quot;
In particular I would not recommend asking questions, at least not in an initial email. I would especially avoid questions such as &quot;would you find this of interest&quot;, which can be very awkward to answer. As you said, you want to avoid looking like you're asking them to serve as a reviewer or supervisor.
That said, in my opinion, sending a paper and indicating that you'd welcome feedback is a perfectly fine thing to do. I've got some interesting feedback this way as a grad student, and more recently I've responded to similar emails coming from students.
Good luck!