I have a zip archive with several files. Zip can password protect individual files (not just the whole archive) and I have such a protection on one file.
How can I extract this file if I no longer remember the password?
So there are a few possible answers.
First, if the zip actually doesn't have a password on it then you will be able to extract the pdf files no problem. then the issue becomes opening the pdf files with a password you forgot. Google "pdf password" and you will find some links to software that can remove the password, the one I have used costed $30.
Second, you are mentioning the zip, because you probably can't unzip it. If this is the case, then there actually is a password on the zip file. Windows natively can't recognize this. You need to install 7Zip, Winzip or WinRar. any one of them will prompt you for the password that you don't even know is there.
If you have one of those programs and the Zip file really doesn't have a password and it still won't unzip, then you still need to run a password removal tool on the zip file even though it doesn't have a password.
If none of that works, then you have a corrupted zip file. Not sure that's recoverable using basic tools.