feat(pdf): add optional output stream parameter to FromMemory overload

- Updated Pdf.FromMemory(MemoryStream) to accept an optional outputStream parameter
- Ensures flexibility when reusing existing MemoryStreams for output
- Added conditional nullability support for .NET builds

EnvelopeGenerator.PdfEditor/Pdf.cs

@@ -18,9 +18,13 @@ namespace EnvelopeGenerator.PdfEditor
             return new Pdf<MemoryStream, MemoryStream>(new MemoryStream(documentBytes), new MemoryStream());
         }
 
-        public static Pdf<MemoryStream, MemoryStream> FromMemory(MemoryStream stream)
+        public static Pdf<MemoryStream, MemoryStream> FromMemory(MemoryStream stream, MemoryStream
+#if NET
+            ?
+#endif
+            outputStream = null)
         {
-            return new Pdf<MemoryStream, MemoryStream>(stream, new MemoryStream(), disposeInputStream: false);
+            return new Pdf<MemoryStream, MemoryStream>(stream, outputStream ?? new MemoryStream(), disposeInputStream: false);
         }
     }