Depending on your skill level, these resources are widely considered some of the best available online:

Moving beyond the pentatonic scale to major, minor, and modal scales opens up new harmonic possibilities for improvisation.

The Ultimate Guide to Guitar Theory by the National Guitar Academy covers scales, chord construction, and musical keys in a structured format.

The Basic Music Theory Essentials by Relax and Learn Guitar covers fundamental physical skills alongside entry-level theory.

To truly become "better," focus on these four pillars often found in top-tier PDFs:

Keep your lessons on your phone or tablet for practice anywhere.