Researchgate encourages you to upload pdfs of your papers to their site. But I would sound a note of caution--just because they invite you to do it, does not mean it is legal. I have been coming across embargoed, preprint and other papers on Researchgate via Google Scholar search.
Many of these papers are most likely still behind the pay wall of the journals they are published in. As such, the authors should not be making them freely available over the internet! Check before you upload or you may be burning some bridges at those journals.