Please note that not all publications have a DOI and that DOIs are not mandatory. ResearchGate assigns DOIs only to unpublished research on request, see https://help.researchgate.net/hc/en-us/articles/14293044585745. If you mean how to get a DOI for a paper you have already published, see these discussions for recommendations:

https://www.researchgate.net/post/How_to_generate_an_DOI_for_publications/1

https://www.researchgate.net/post/Getting_DOI_for_individual_papers/1

https://www.researchgate.net/post/How_can_I_get_DOI_for_my_paper/1

https://www.researchgate.net/post/How_can_I_get_a_doi_for_an_article/1

https://www.researchgate.net/post/Adding_DOI_to_documents/1

https://www.researchgate.net/post/Is_there_special_app_to_generate_Doi/1

Hello Nermin Younes

Do you mean to ask how to find doi of an article you downloaded? What do you need it for? Do you need it for a reference in your own paper? Those are often a available from databases a pnd search engines such as Pubmed.

https://pubmed.ncbi.nlm.nih.gov/