Realizing this is an old post, I want to offer my knowledge for future inquiries. I read a lot of posts and found no advise to my specific question, "Why does attachment options like 'Open in iBooks' not show in mail while previewing?" I, actually, had to contact my IT department AND Apple to get this issue resolved.
This is specific, but not inclusive, to those whom access their work email (Which is normally secure. Such as Exchange servers or other VPN required sites) through their iDevice, Droid, Chrome App, etc. Exchange and other networks have implemented source coding programs targeted at keeping data secure. This can keep us from opening attachments in iBooks, Acrobat, Microsoft Office, etc. Your provider can encrypt this code into your device (s) when you request access to your section of the server (ie: your work email). You may not have been aware but a lot of apps do this.
You can, most times, circumvent this by forwarding your work email to your personal email. The only except to this (that I have found thus far), is that if the writer or publisher of the document has encrypted the actual document itself (Such as "Read Only" or "Uneditable").
This may seem like a hack or loop, but it isn't. Allowing you to email it to yourself still protects their interest (Copyright infringement lawsuits). And it can allow you to do your job (just remember that YOU, now, ARE LIABLE FOR YOUR COPY & more than likely, being monitered).