[vb 2010 express]: open and save pdf file with password

Hi all.
I just joined this forum. Greetings to all.

I am an "old" amateur programmer VB5 now, inevitably, would move to Visual Basic 2010 Express.
To begin with I have to do a little project that allows me to change the password for a pdf file.
Warning: I do not need to open it and view it or change it!
What I would do is this:

I select the file
thne decryption key with the password that I know
then crypto with the new password
save it with the same name

all without the user having to type or see anything (except for the old password and the new).

I think that in the native vb library there is nothing and that I should use some external library ....

Can you tell me something, and give me a small guide on how to use it

thanks