There is no basis to say that Dropbox is insecure. It is a fully password protected cloud server. I assume that the information you are using is not sensitive in the military sense or commercial context therefore Dropbox is an easy way to provide you with the capability you seem to be after.
With regards to software on the iPad. I would recommend goodreader. It links to your Dropbox account and downloads (syncs) the documents to the iPad allowing the information to be used offline. When a new Airac is released just connect to the wifi/3G and hit sync. Goodreader will do the rest.