What is the best format for preparing instructions?

Question

Books

When I was going to create instructions for the software, I initially chose the CHM format, because this format does its job admirably. The only trouble is that it looks very unsightly. Is it possible to create the same convenient instructions, but in a modern cover?
Interested in:
Cataloging
Search
Offline
Ease of reading
Design flexibility
Viewing without additional software (for example, through a browser)
UPD.
PDF is very heavy. The instruction will be updated very often and it will be difficult to pull it out.
Hotkeys are not an option, because the instruction for users is 30-40 years old.
And one more thing... the instruction will most likely develop into an offline wiki. Big and plump

Answer 1

With CHM I had problems on win 8 - some users got a blank page instead of help.
If you want design - I recommend using PDF - it can do everything that you described

Answer 2

PDF is very heavy. The instruction will be updated very often and it will be difficult to pull it out.
Hotkeys are not an option, because the instruction for users is 30-40 years old.
And one more thing... the instruction will most likely develop into an offline wiki. Big and plump

Answer 3

https://github.com/gollum/gollum