Here are the 3 different types of password protection I’ve suggested:

1. Password protect each file and have a separate box for each file.

On each page with a file, there would be a password box for each file so they can download them. You could use different passwords for different files or use the same password for all of them.

PRO: There is more control over the file and password. You can change passwords separately for each file if need be. 
CON: You would end up with password boxes in the middle of pages where it might not look the most natural.

2. All protected files would link to a password protected download page

Rather than creating passwords for each file, you could create one password for a single page with links to download the files. All the protected files would be available on that page so anyone with the password would have access to all the files.

PRO: Protects all the files in one place, making it more convenient to protect and download all the files you need at the same time without having to search throughout the site.
CON: If breached, all the files would be compromised at the same time.

3. Low resolution image publicly available as well as a password protected high resolution file

Clicking to view a file would open a small, low resolution preview image so they can see the gist of the file without being able to see enough details to steal it. There could be a password box on the page as well so they can download the hi-res PDF if you give them the password.

PRO: Casual viewers can see something there, but only people who have been given the password can see all the details.
CON: Not all files would look good as low-res images (like documents without pictures).

Method 1:

Password is: passwordtest

Method 2:

Click here to be directed to the download page.

Method 3:

Grey Rain System