Using Paper under review as a Scientific Report in PhD application I have submitted a paper at a conference and it is currently under review. Can I use this paper as a 'scientific report' (asked to assess candidate's writing skills) for a Ph.D application ?
So long as this is for an internal evaluation process that doesn't involve any form of "publication," it's hard to see how this could be a problem. You retain rights as the author of a paper, especially if it hasn't been published. 

If you have concerns over whether it is allowed, you can always ask the conference organizers if it's acceptable. On the other hand, if you are concerned whether the department you are submitting to will accept it, ask the department.