This appears not to be an NOF problem. However, someone might find this post useful.
I have played with this and found that Chrome browser on Android devices does, in fact, recognize the .htaccess login. It will download
GBWhatsApp .html, .doc, .docx, .txt files and maybe others, but will not download and open a .pdf file from the htaccess protected folder. This may require a plug in or something for the Chrome browser. Or, like what was suggested, to find another folder-protect-app that is compatible with Chrome.