The pfile can never be modified by RDBMS, so the changes will never be written in it automaticaly. Whether the changed values of parameters will persist even after you bounce the database depends on wether the instance started with spfile or not.
If the database was started without spfile then no, the changes made with ALTER SYSTEM SET .... will be lost after reboot. If it was started using spfile, then unless you specify ".... SCOPE = MEMORY" at the end of your ALTER SYSTEM command, the changes will be stored in spfile and will take effect when you restart your database.
Jurij Modic ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?