You need the Dir- statement to run through all files in the directory:
Dim PathLocation As String
Dim fileName As String
Dim filePath as String
PathLocation = "C:\Test\"
If Not IsNull(PathLocation) Then
Set objAttachment = objMailDocument.CREATERICHTEXTITEM("strFileAttachment")
fileName = Dir$(PathLocation & "*.*", 0)
Do While fileName <> ""
filePath = PathLocation & fileName
Set objEmbedObject = objAttachment.EMBEDOBJECT(1454, "", filePath, "")
fileName = Dir$()
Loop
End If
To run through e.g. just PDFs replace the *.*
in the code above by *.pdf