If your document is under the solution you will need to copy it first to TEMP project and then to copy it to the other solution that will include this TEMP project.
If your document is under a project that is under a solution, you will need then to temporary include that project under the target solution and then perform the copy.
Answer for How can I copy document item between solutions
Author: Rami.Azulay
Expert in deployment of lifecycle management system into Medical Device vendors who seeking warranting successful Medical Device Auditing and Regulatory Compliance through Process Automation Tools It is no secret that Medical Device companies incur much costs and organizational stress relating to creating and maintaining their Technical File, DHF or ISO documentation, especially before first time submission or external auditing by notified bodies. https://il.linkedin.com/in/ramiazulay