DOIs are not identifiers for publications, as often assumed, but identifiers for digital objects (the name says it). These can be files with publications, but also files with data or other digital objects. Therefore, a printed publication for which no file (usually PDF) exists, cannot have a DOI.

@Wolfgang R. Dick, I did make a digital pdf by scanning the document (sharing the link below). Now I want to add a DOI to it so that it's availability increases publicly.

https://www.researchgate.net/publication/375238791_Comparative_Efficacy_of_Therapeutic_Plasma_Exchange_and_Immunosuppressive_Therapy_in_Systemic_Lupus_Erythematosus_A_Case-Control_Study

See Rob Keller's reply to https://www.researchgate.net/post/Can_I_use_ResearchGate_DOI_for_an_article_uploaded_in_RG_which_is_published_elsewhere_in_a_Journal how to create a DOI.

Thank you Wolfgang R. Dick for the reference. I think I will use zenodo too 🙂