Edit a text file on Android (free, no ads)

This really shouldn't be so hard. I've tried gsearching and found endless hideously annoying false leads. Half the time a "recommended solution" doesn't do what I'm asking for, in one way or another. Some of these things expect you to pay... yes, money! And for me ads are not and never will be acceptable, in any circumstances whatsoever. And many of them don't actually have any way to enable you to open a file, as with a typical computer program: i.e. a "File" menu with "New, Open, Save, Save as", etc. Nor does tapping on an existing downloaded file work: the only offered apps to open an .htm file, for example, are "Chrome" and "HTML viewer" currently. So with a vast number of these apps it appears impossible to actually open an existing file! There may be ways of adding to the number of apps suggested for "open with" a particular file type... but the whole point is that I want to open files with extensions like ".htm" and possibly ".css" and who knows what else. Just let me navigate in the file system and choose my file, For Frederick's Sake. I finally thought I'd found a solution: "OldSchool Editor". I managed to open an .htm file in it, and even edit and save the text. And then open the tweaked file in a browser app. But for some reason that app is afflicted with a totally crippling typing latency, making it unusable: in the edit page, with an .htm file of 11kB, I press a key, and then it takes approximately one second before the screen adjusts, sometimes longer. Another suggestion made is to use Google Docs. But that means interacting with the Cloud, as their docs are then kept there, and you can only edit things while online. I must be able to do stuff while offline. Like Notepad++ on a computer. Even if I tried Google Docs I'd be pretty unlikely to be able to manage the actual text, in the case of an .htm file. Any solutions to this? It seems quite a basic thing to want to do. **PS: "this isn't for software recommendations".** I realise there may in theory be a more appropriate forum for this question, indeed.
But firstly I think that in practice that other forum is for computer platforms, not mobiles. I need to tax the brains of none other than **android enthusiasts**.
Secondly, I'm not asking for an app which is comparatively "better" than another. Just ANY app which does this thing I want to do.