For that your userform's Command Button would use code along the lines of:
Private Sub CommandButton1_Click()
Dim DocTgt As Document, DocSrc As Document
Set DocTgt = ActiveDocument
If Me.CheckBox1.Value = True Then
Set DocSrc = Documents.Open(FileName:="Path & FilenameA.docx", _
ReadOnly:=True, AddToRecentFiles:=False, Visible:=False)
DocTgt.Range.Characters.Last.FormattedText = DocSrc.Range.FormattedText
End If
If Me.CheckBox2.Value = True Then
Set DocSrc = Documents.Open(FileName:="Path & FilenameB.docx", _
ReadOnly:=True, AddToRecentFiles:=False, Visible:=False)
DocTgt.Range.Characters.Last.FormattedText = DocSrc.Range.FormattedText
End If
If Me.CheckBox3.Value = True Then
Set DocSrc = Documents.Open(FileName:="Path & FilenameC.docx", _
ReadOnly:=True, AddToRecentFiles:=False, Visible:=False)
DocTgt.Range.Characters.Last.FormattedText = DocSrc.Range.FormattedText
End If
If Me.CheckBox4.Value = True Then
Set DocSrc = Documents.Open(FileName:="Path & FilenameD.docx", _
ReadOnly:=True, AddToRecentFiles:=False, Visible:=False)
DocTgt.Range.Characters.Last.FormattedText = DocSrc.Range.FormattedText
End If
End Sub
The above code assumes your userform's:
• Command Button is named CommandButton1
• Checkboxes are named CheckBox1 - CheckBox4
You will, of course, need to supply the correct filepaths & filenames.
As coded, each inserted document is added to the end of the active document.