Windows: Add feature to save pdf in menu

Elwig/Documents/Document.cs

@@ -99,6 +99,8 @@ namespace Elwig.Documents {
         }
 
         public async Task Generate(IProgress<double>? progress = null) {
+            if (_pdfFile != null)
+                return;
             progress?.Report(0.0);
             if (this is PdfDocument) {
                 // nothing to do