For security reasons UWP apps cannot open or save a file unless the user selects it...that's what the FilePickers are for. The only exception is your application's local storage folder. You can retrieve a list of files from the folder and present them to the user for them to select:
Windows.Storage.StorageFolder LocalStorageFolder = Windows.Storage.ApplicationData.Current.LocalFolder; IReadOnlyList theFileList = await LocalStorageFolder.GetFilesAsync(Windows.Storage.Search.CommonFileQuery.OrderByName);
An alternative is to group all of the xaml files into one file so the user just needs to select one file to retrieve all of the xaml.
You say you have 20 text boxes in each of the 20 xaml sheets...is the user creating those 20 xaml sheets themselves or with another app independent of your application? If so then I suggest that when the user selects a xaml sheet you cache a copy of it in your localfolder and when the user wants to open a xaml sheet you first present a UI containing the list of files from your localstorage plus a button to launch a FilePicker for your app to "consume" a new file.