The SharePoint document library is integrated onto main parent table. The quick view form is created on this main parent table. The related Documents subgrid id added onto the quick view form. The quick view form is added onto the related child table's main form.

The list of documents are displayed in Documents subgrid of quick view form on child table's main form, but unable to access/open the documents from the subgrid.

The response received from Microsoft Support team is as follows:

Embedding the document sub grid in a related entity quick view form and configuring the grid to access documents from its parent entity is currently unsupported.

It is nice feature to have it and it is may be useful for many users.