This button will open the first PDF found in a folder which you specify within the button:
After adding the button, edit it to change the line specifying the folder you want it to look in.
The script inside the button looks like this:
Option Explicit
Function OnClick(ByRef clickData)
Dim cmd, folderEnum, folderItem, folderPath
folderPath = "C:\Folder Path\With\A PDF In It"
Set cmd = clickData.func.command
cmd.deselect = false
cmd.ClearFiles
Set folderEnum = DOpus.FSUtil.ReadDir(folderPath, False)
Do While (Not folderEnum.complete)
Set folderItem = folderEnum.Next
If (UCase(folderItem.ext) = ".PDF") Then
cmd.RunCommand folderItem.RealPath
Exit Do
End If
Loop
End Function