Check again with the process monitor tool from where the pdf PreviewDetails are being read now, as you have done yesterday.
Until the PDF PreviewDetails property list is not read from the HKEY_CURRENT_USER\Software\Classes\SystemFileAssociations\.pdf or, in your specific case, HKEY_CURRENT_USER\Software\Classes\RedlinePDFfile, you need to delete all the other sources.
Or, probably better, you may change the HKEY_CLASSES_ROOT\.pdf default value from the current RedlinePDFfile to AcroExch.Document, and set again the details pane properties from the manager.
After you added that "Sent" named property, you installed a new PDF reader that configured itself as the default reader, but then you have manually override the default reader back to Acrobat. Because of a bug in the manager, the PreviewDetails list is being saved to the newly installed reader keys, instead of the correct place of your current default PDF reader that is Acrobat.