Azure Information Protection Documentation Update for December 2018

kaibisnes SharePoint IRM-protected libraries currently use an older format for protected PDFs, and to the best of my knowledge, Adobe Reader has always supported this older format.  The updates mentioned in this blog post are for supporting the latest (ISO) format for protected PDFs.  If you're having problems opening PDFs from IRM-protected libraries, use the technical support resources for the vendor.  On the Supported PDF readers for Microsoft Information Protection page, we have a section that lists readers that support previous versions for protected PDFs.  You might want to try some of the alternative options - a quick web search should provide information about licensing and downloading.