By making sure that Adobe Acrobat Distiller generates Acrobat 4.x-compatible
documents you also ensure that no unknown objects will be included in
the document:
In Adobe Acrobat Distiller, choose Settings
> Edit Adobe PDF Settings and select the General tab.
In the File Options area, make sure that Acrobat 4.0 (PDF 1.3) is selected in the Compatibility list.
Click OK to save the changes to the Adobe PDF Settings.
Re-create the PDF document using the new Adobe PDF Settings.