我找到了两种解决方案：
The first one正在使用内建的Shell()函数。这应该会自动解析相对路径（相对于应用程序目前的工作目录）：

Public Sub StartExeWithArgument()
    Dim strFilename As String

    strFilename = "../folder/file.pdf"

    Call Shell(strFilename, vbNormalFocus)
End Sub

The second one使用Shell.Application COM对象，基本上与第一个COM对象的操作相同。

Sub runit()
   Dim Shex As Object
   Set Shex = CreateObject("Shell.Application")
   tgtfile = "../folder/file.pdf"
   Shex.Open (tgtfile)
End Sub

如果你从ThisWorkbook.Path和你的相对引用开始，然后在相对引用中为每个"..\"修剪一个层，你就会得到路径。

Function RelativeToAbsolutePath(ByVal RelativePath As String) AS String
    Dim TempStart AS String, TempEnd AS String
    TempStart = ThisWorkbook.Path
    TempEnd = RelativePath
    If Left(TempEnd,1) = "\" Then TempEnd = Mid(TempEnd,1)

    RelativeToAbsolutePath = ""
    On Error GoTo FuncErr

    While Left(TempEnd,3)="..\" AND InStrRev(TempStart,"\")>0
        TempStart = Left(TempStart,InStrRev(TempStart,"\")-1) 'Remove 1 layer from Workbook path
        TempEnd = Mid(TempEnd,4) 'Remove 1 instance of "..\"
    Wend

    RelativeToAbsolutePath = TempStart & "\" & TempEnd 'Stitch it all together
FuncErr: 'You may want a DIR(..) check to see if the file actually exists?
End Function

然后可以使用Shell打开它