I have recreated your folder on my network and provided you have the backslash at the end of the path (your's doesn't) it works OK
Code:
Option Explicit
Sub AttachPDFs()
Dim olItem As Outlook.MailItem
Dim olAttachments As Outlook.Attachments
Dim strPath As String
Dim strFile As String
Dim i As Long
Dim oFrm As New UserForm1
With oFrm
.Caption = "Select files to attach"
.Height = 272
.Width = 240
.CommandButton1.Caption = "Continue"
.CommandButton1.Top = 210
.CommandButton1.Width = 72
.CommandButton1.Left = 132
.CommandButton2.Caption = "Cancel"
.CommandButton2.Top = 210
.CommandButton2.Width = 72
.CommandButton2.Left = 18
.ListBox1.Top = 12
.ListBox1.Left = 18
.ListBox1.Height = 192
.ListBox1.Width = 189
.ListBox1.MultiSelect = fmMultiSelectMulti
'If the files are always in the same folder you could set strPath to that folder e.g.
strPath = "X:\_sponsor_folder\"
strFile = Dir$(strPath & "*.pdf")
While Not strFile = ""
On Error GoTo err_Handler
.ListBox1.AddItem strFile
strFile = Dir$()
Wend
.Show
If .Tag = 1 Then
Set olItem = Application.CreateItem(olMailItem)
Set olAttachments = olItem.Attachments
For i = 0 To .ListBox1.ListCount - 1
If .ListBox1.Selected(i) Then
olAttachments.Add strPath & .ListBox1.List(i), _
olByValue, 1
End If
Next i
olItem.Display
End If
End With
Unload oFrm
lbl_Exit:
Set olItem = Nothing
Set olAttachments = Nothing
Set oFrm = Nothing
Exit Sub
err_Handler:
MsgBox Err.Number & vbCr & Err.Description
Err.Clear
GoTo lbl_Exit
End Sub