You can you modify the PL/SQL Developer shortcut and add the DebugSQL parameter. For example:
"C:\Program Files\PLSQL Developer 12\plsqldev.exe" DebugSQL
The debug.txt file is generated in the %APPDATA%\PLSQL Developer 12 directory (e.g. C:\Users\\AppData\Roaming\PLSQL Developer 12).