you need to include the file extension ..
Try this:
Option Explicit
Private Declare Function ShellExecute Lib "shell32.dll" Alias "ShellExecuteA" (ByVal hwnd As Long, ByVal lpszOp As String, ByVal lpszFile As String, ByVal lpszParams As String, ByVal lpszDir As String, ByVal FsShowCmd As Long) As Long
Public Function OpenFile(ByVal FileName As String) As Long
OpenFile = ShellExecute(0, "Open", FileName, "", "C:\ALL_pdf_Cut_Sheets\Actuators\", 1)
End Function
Sub file_get()
OpenFile "C:\ALL_pdf_Cut_Sheets\Actuators\ms8105a1008.pdf"
End Sub
In your form, make the action of the click event for your button call file_get