D'habitude, c'est très utile et me sauve de beaucoup d'actionnes ennuyeuxses, comme copier et coller chaque lettre d'un/depuis un autre document.

"action" is feminine, like all words with the noun suffix -tion => ennuyeuSES
"copier-coller" would also be right
You can use "depuis" to make clear your "de" is a "from"

En revanche, quelquefois, j'oublie que ce clavier est activé et (je) taper quelques/certaines lettres absurdes sur mones documents.

"quelques" would emphasize "not many"; "certain(e)s" would emphasize "some of them, not others", but both could be used here
I suppose it happened to you when typing more than one document, so I'd pluralize this one => mES documentS

J'ai (de) la chance d'avoir un raccourci clavier pour le changer.

There would be a small nuance of meaning between "avoir DE la chance" and "avoir la chance"; tje second rather means "having the opportunity/the privilege", the first is "being lucky" in the proper sense of the word, but both would be right