Viewing/Opening Multiple Documents

I upgraded my Reader to the Acrobat DC and now it will only open one document at a time.  For clarification I understand how to manually open a new tab so multiple documents are open but it won't do it automatically.  For example if I have one document open and then click on a PDF attachment in an e-mail the document won't open.  I have to either close down the existing document or save the attachment to file and open it manually.  How do I fix this.  Previous versions of Reader never had this issue.