I don’t know if MongoDB is “best” for such a portal, but the following might be the best argument in favor of MongoDB vs. MySQL (both of which I use in web design):
Most of your data entities will be largely self-contained. A user profile can easily be expressed as one document, without requiring relational normalization. Likewise, for your job offerings, there will be only one relational element: the one-to-many relation between a job offerer and multiple job offerings.
One question will be: which system makes it easier to do the queries and searches you plan?
Model your system in both environments and see which you find more comfortable.
Experience, even a small amount of experience, beats recommendations every time!