What software should i use?

You can't. The only way is to use a notation software like Finale, Sibelius or any other.

hhm.so how to convert the pdf file into .mus file?

There are a few programs that can scan pdf files and convert it into .sib or .mus files I believe, never used them myself though.

I have found some applications to convert .mus to .pdf but cant find .pdf to .mus...
so,usually,how if you want to edit sheetmusic that is in .pdf file extension?

I have found some applications to convert .mus to .pdf but cant find .pdf to .mus...
so,usually,how if you want to edit sheetmusic that is in .pdf file extension?

You can't.

so why i see that many many people save the music sheet into pdf file?
though it cant be edited...

so why i see that many many people save the music sheet into pdf file?
though it cant be edited...

How many people do you know have Finale AND Sibelius AND Music Sheet Maker AND the list could go on.

Or how about a universal file called PDF which runs on the ever popular Adobe Reader which every computer can read?

=]

*edit*

Also the fact that PDF can't be edited means that copyright laws can be withheld.
I wouldn't want my sheet music get changed by some anonymous person who writes his name instead of mine, then distributing it claiming it is his.

oo.okay2.
btw,can finale notepad 2009 used to convert .mus into .pdf?how?

You would need primopdf or any other converter: www.primopdf.com/ Just download it and when you going to print in finale, change the printer for primopdf.

When you make the .MUS>>.PDF convertion it's like printing the file, so now it will be like an image.

If you wanna edit some little detail, and making the whole song from scratch in .mus is not worth the time.... you could edit the pdf with, for example, Photoshop.

Dunnow what you wanna do though...