How to keep local documents on iPad

I travel in parts of the world with little or no internet, and want to take my iPad with a collection of travel documents, books, and offline emails. On a recent trip I carefully downloaded books and PDFs into the Books app, various documents into iCloud Drive, and opened all my email to make sure they were downloaded to Mail. To my disappointment, throughout the trip the iPad regularly deleted these documents and the information was often not there when I needed it. I would find myself with the picture of a book cover with no contents, or a cloud symbol instead my plane ticket. I can see no logic to how this was done. A PDF downloaded on the day of departure is gone, but useless years old documents are still there. Not even the emails were still there. After leaving home I had sorted my emails into folders which causes the contents and attachments to be discarded. 

Is there a way to fix this? I cannot find any. Alternatively, is there some part of the iPad that will not randomly delete itself when you disconnect from the internet? I would be willing to convert everything to PDF and stash it somewhere.