From Notepad++ Wiki
How to open only one file per instance of Notepad++
There are two ways to coerce Notepad++ to behave like vintage Notepad:
- The simplest way is to create a zero length file named asNotepad.xml beside your notepad++.exe .
- The second way is to launch Notepad++ via Dos prompt, by giving 3 arguments + the file name to open :
notepad++ -multiInst -notabbar -nosession d:\file2open