Please forgive me if I got you wrongly.
Refer the below pic.
In there, first I have added the file (User Manual.pdf) by right clicking on the My Computer from the Tree.
Then from the Installer properties, under category Source Files, selected the same file (Project View) and then selected [Desktop Folder]. Click the Right Arrow in between these two sections. You will see the file added under [Desktop Folder].
When you install using the installer created this way, the file will be installed as per the settings.
Hope it helps !