For detailed direction on how to open and reply to an encrypted email for the first time you can find a step by step tutorial at: http://support.emailpros.com/933677-How-to-open-an-encrypted-email-for-the-first-time
Please be aware that you will need to download Adobe Reader to view your encrypted email. You can download Adobe Reader at: https://get.adobe.com/reader/
If the sender attached a file to the encrypted email, it will show up as an embedded attachment on the top left of the PDF file. If you are using a PDF reading program other than Adobe Reader (e.g. Acrobat, Preview on Mac), you will not see the attached file.