可能的原因是缺少Adobe Reader XI的一些组件。可以尝试修复Adobe Reader XI安装并更新到最新版本。如果问题仍然存在，则需要卸载Adobe Reader XI并重新安装。

以下是使用VBA代码在Adobe Reader XI中打开评论的示例：

Sub OpenComments()
    Dim AcroApp As Acrobat.CAcroApp
    Dim AcroAVDoc As Acrobat.CAcroAVDoc
    Dim AcroPDDoc As Acrobat.CAcroPDDoc
    Dim AcroPDPage As Acrobat.CAcroPDPage
    Dim AcroPDAnnot As Acrobat.CAcroPDAnnot
    
    Set AcroApp = CreateObject("AcroExch.App")
    Set AcroAVDoc = CreateObject("AcroExch.AVDoc")
    If AcroAVDoc.Open("path\to\pdf\File", "") Then
        Set AcroPDDoc = AcroAVDoc.GetPDDoc()
        For i = 0 To AcroPDDoc.GetNumPages() - 1
            Set AcroPDPage = AcroPDDoc.AcquirePage(i)
            For j = 0 To AcroPDPage.GetNumAnnots() - 1
                Set AcroPDAnnot = AcroPDPage.GetAnnot(j)
                If AcroPDAnnot.Subtype = "Text" Then
                    AcroPDAnnot.OpenPopup()
                End If
            Next j
            AcroPDDoc.ReleasePage AcroPDPage
        Next i
        AcroPDDoc.Close
    End If
    AcroAVDoc.Close (True)
    AcroApp.Exit
    Set AcroApp = Nothing
    Set AcroAVDoc = Nothing
    Set AcroPDDoc = Nothing
    Set AcroPDPage = Nothing
    Set AcroPDAnnot = Nothing
End Sub