Almost all of the Postdoctoral positions requires to attach publications while contacting. Can a manuscript under review be sent to address this? I have submitted my PhD thesis and am searching for postdoc positions. However, every position has something on the lines of "Postdoctoral applicants should also attach your most important publications." I had published a work from my PhD two years back, while one is under review at the moment. The one under review being the accurate representative of the novelty of work done in my PhD.

My question is: Can I send the manuscript under review as an example of my work? Is there an accepted/legal way of doing it, like sending the submitted copy generated by the journal?
You can send it if you clearly label it as under review and not yet accepted. Name the journal reviewing it as well. 

I don't see any problem with this as long as you are honest about its current status. Of course you should also include other publications if available. But in your labeling of this paper you can give your personal assessment of its importance in your work. People can do their own evaluation.