This is probably due to the fact that NotePad is using UNICODE. Try this
int copied = SendMessage(hwndEdit, WM_GETTEXT, (WPARAM)bufferSize, (LPARAM)textBuffer);
printf("Copied %d chars.\n", copied);
to find out how many character your callee thinks it copied. Try the following to print UNICODE text:
const int bufferSize = 1024;
wchar_t textBuffer[bufferSize] = "";
int copied = SendMessage(hwndEdit, WM_GETTEXT, (WPARAM)bufferSize, (LPARAM)textBuffer);
printf("Copied %d chars.\n", copied);
printf("getting text:\n");
wprintf(L"%ls \n", textBuffer);
If you are using Visual Studio, it might be worth a try to go into your project options (Right click project -> Configuration Properties -> General -> Character Set) and set it to ANSI ("Not Set").