It might be helpful to have a search box within the Invoice / Quote View tab.
That whay you could easily search for an existing invoice / quote without leaving viewing tab.
I think it would be less time consuming and you would not need to go through the whole list of invoices / quotes if you already know the number or the clients name.
Something like this.
The Search function always applies to data in a table, such as a tab listing. When you are viewing a transaction like a sales invoice, you are not looking at a table, but at presentation (by a theme) of one record in the database. So there is nothing to search, that is, there can be no subsets of the data you are already viewing. You must return to the original table to have more data available to search.