See "How do I respond to full-text requests?" in the help page https://help.researchgate.net/hc/en-us/articles/14292986311825-Discovering-and-requesting-research:
"If a privately stored copy of the full-text is available on ResearchGate, you can choose the version of the stored publication you wish to upload and select Share this full-text. You can choose to make the full-text public by selecting Add as a public full-text, or send it privately to the researcher by selecting Send in message. Alternatively, there is an option to Share different full-text, in which case you can select a file from your computer."