search encrypted documents

@ https://helpx.adobe.com/acrobat/using/searching-pdfs.html There is a note stating:    If documents are encrypted (have security applied to them), you cannot search them as part of a multiple-document search. Open those documents first and search them one at a time. However, documents encrypted as Adobe Digital Editions are an exception and can be searched as part of a multiple-document search.   I am doing advance search however those PDFs encrypted as Adobe Digital Editions are not included as part of a multiple-document search. How can I include these? The message is stating: "search has skipped 6 files because either these files are corrupt or you do not have permission to open them".