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Kindles have password protection but I've never used it so I can't tell you how secure it is. They're not very good, however, for PDF files. They'll work with PDF if the text can be flowed but things like two column pages and tables and charts are less than ideal. They are fine for text files and PDF's can be made of text files that work well on a Kindle.
I'm not aware of password protection on other ereaders although some may have it. Most don't work as well with PDF as Kindles do.
There are larger e-ink ereaders with Android, usually version 4.? available. I don't know much about them but they might be usable with PDF and support passwords. They come in sizes up to about 13". 9" models aren't too unusual. I've never seen one of these things and I don't really know much about them except that they exist. Some do have access to Google Play, which means there should be plenty of apps for them. It might even be possible to read encrypted files.
I'm not sure how much help all this is but it might give you some things to look into.
Barry
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