"The pdf.js library could not be found." (PDF module)
The path will be like this /sites/all/libraries/pdf.js.

pdf.js is a FOLDER not a FILE. It is very strange to have a folder with a DOT in the name.

I have attached a screenshot on the folder hierarchy that works.

SUMMARY:

The folder in your libraries folder must be named: pdf.js, you will have to rename the top folder of anything you download to pdf.js (in this instance pdf.js is not a FILE, but the container of the library)
See screenshot for the correct file hierarchy.