Are there any tools to automatically search and download literature in a given .bib file? I'd like to download automatically tons of papers listed on .bib file so I'm trying to find a way to facilitate the process by, e.g. I import or input to a tool a certain .bib file which contains a list of literature, the output is the literature's .pdf file. Any idea to do so?
In contrast to the freemium/proprietary tool suggested in the answer by andreas, I propose a FOSS solution that works reliably for this specific task.

Assuming you are connected to a network that provides permissions to legally download the required content, the cross-platform reference manager,JabRef has integrated fetchers to download full-texts for the entries in your library. Import your bib file into Jabref (maybe just start a new library for this task to keep things clean) and select the desired number of entries for which full-text PDFs are needed. Then, from the Quality pull-down menu, select Lookup Fulltext documents.

However, you still have to manually confirm all downloads. From the user's perspective, this requires just a bunch of clicking the OK button in the same on-screen co-ordinate (for pop-ups that appears sequentially for each citation entry). Jabref does all the heavy-lifting and correctly downloads the desired PDFs of the full-text and link each of them appropriately by matching to their relevant citation entry in the library.

The latest master build of Jabref is recommended, since there were some recent fixes to its IEEE fetcher (i.e. if IEEE matters to the OP).