Why don't you want the file locked - wat's the background to this question?
Maybe you cud create a wrapper applet for yr PDF viewer so whenever you open a file, it either marks the file as read-only or opens the file 4 writing - thus the viewer will have no choice but to open the file as read-only.
When the viewer exits, the file can be restored to normal.
But again, this depends on yr situation & yr problem.