単純に組み合わせて以下でどうでしょうか
Private Sub microbe_Click()
ChDrive "R"
ChDir "R:\○○\××\△△"
Filename = Application.GetOpenFilename("PDF Documents,*.pdf")
If Filename = False Then Exit Sub
Set myShell = CreateObject("WScript.Shell")
myShell.Run ("AcroRd32.exe /t " & Filename)
Set myShell = Nothing
End Sub
グロじゃナイヨ~!、ダイアロ~グ、、、
Sub GetOpenFilenamePDF()
Dim xFileNames As Variant
Dim nn As Long
xFileNames = Application.GetOpenFilename(filefilter:="PDF Documents,*.pdf", MultiSelect:=True)
If (Not IsArray(xFileNames)) Then
MsgBox "キャンセルされました!"
Exit Sub
End If
For nn = 1 To UBound(xFileNames)
MsgBox (nn & " 番目のファイル:" & xFileNames(nn))
Next
End Sub