Can I protect the file with a password, so I can open that readonly without password?
Thanks
You can do that with a section protected against modifications.
For an existing file: ctrl+A (select all) → insert → section etc.
Oh yes, it works! Unfortunatelly, not in Impress, but sufficient.
Menu/File/Propierties/Security
Here you can only lock the revision marking
With LibreOffice version 3.5.7.2 and 3.6.3.2
Try it, it protects only the record changes