A DOI is a unique permanent number used to identify digital content which stands for Digital Object Identifier. For instance, a DOI can be generated for a journal paper you have published. DOIs provide a reliable link to your work online, making it easy to be cited by others. DOIs can be generated for both published and unpublished works. Publishers generate DOIs for papers they publish. A DOI also can be generated by other websites as well, for instance ResearchGate. If you add a contribution to your account in ResearchGate, the website generates a DOI for the file you have uploaded. When you have a DOI for an existing publication you can just search the DOI number and it takes you the corresponding page.