How to add shortcut to Cinnamon taskbar

A simple GUI method:

Right-click Menu and then click Configure.

Click Open the Menu Editor.

Optionally create a new folder for your custom links. Create a new item that opens the file, using the command, evince /path/to/file.pdf, or whichever PDF viewer you want to use.

Close the menu editor and right-click on your new menu item, selecting Add to Panel.

If you chose to make a new folder in the menu, it exists in ~/.local/share/desktop-directories/ as a file with the extension, .directory. If you chose to make a new menu item, it exists in ~/.local/share/applications/ as a file with the extension, .desktop. These were created by alacarte. They are regular text files; and, now that you know their location, you could do this manually, too.

The rest of the files for the menu are located in /usr/share/desktop-directories and /usr/share/applications.

There is another way to do this besides Christopher's method. I've tested it in Linux Mint 17.3.

Simply find the application in the Menu and drag and drop into the launcher. I struggled with the whole "adding shortcut" thing, until I thought to try that. As a bonus, it brings the app icon along as well.