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.
Submit it as a preprint in ArXiv and cite the corresponding document (I am assuming it has the minimum quality to be submited here). Producing preprints is a good practice (specially for countries with limited access to co$tly journals).
In my opinion: do not cite "submitted to TOP JOURNAL" papers. Only Tyche knows what is going to happen with that submission (e.g. maybe the referee is your arch enemy).