@Vittorio-Zamparella
A solution is just another way of referring to a project (that is, the files that you care about editing).
You have to create a solution using the SolutionHub UI. The SolutionHub UI section of the link I posted explains how to create a solution. Once you have done so, and you have created a connection to OFIS in the solution (the link explains how), you can search for files in the solution by name (optionally by the entire path and name).
I cannot remember if the plug-in creates a hotkey or if I used the NPP UI to create one. Anyway, I type Alt-O to open the “Open File in Solution” dialog box. You can also get to the dialog from the NPP menu:
Plugins–>Open File In Solution–>OFIS - Show
You can get help on how to use the OFIS dialog from the NPP menu:
Plugins–>Open File In Solution–>OFIS - Help