Your terminology is not good. Bookmarks have a specific meaning in Word and you are using this word in a way that doesn't make sense.
It appears you are wanting to store the path to a specific folder. That can be done either by creating a constant at the top of the module or by using a function that each macro calls. The benefit of the function approach is that the code could then be made to adapt to the relative path (ie the files are always in the desktop folder but that actual path will vary for each user). Using a hardcoded constant, the code could be written as follows.
Code:
Function InsertAFile(sFile As String)
Dim sPath as String
sPath = "C:\Users\YongX\Desktop\"
Selection.InsertFile FileName:=sPath & sFile, ConfirmConversions:=False, Link:=False, Attachment:=False
Selection.TypeBackspace
End Function
Sub Add_HT_PD()
InsertAFile "HT PD.docx"
End Sub
Sub Add_HT_US()
InsertAFile "HT US.docx"
End Sub
Sub Add_TF_PD()
InsertAFile "TF PD.docx"
End Sub
Sub Add_TF_US()
InsertAFile "TF US.docx"
End Sub
Sub Add_HT()
InsertAFile "HT.docx"
End Sub
Sub Add_LT()
InsertAFile "LT.docx"
End Sub
Sub Update_Fields()
Application.ScreenUpdating = False
ActiveDocument.PrintPreview
ActiveDocument.ClosePrintPreview
Application.ScreenUpdating = True
End Sub