This is a good question, I'm glad you are asking. The person you care about is lucky to have someone like you who wants to encourage them like you do. Kudos to you for that. We need more supporters in the world who want to help, and seek out to understand how to best help.
For me, I find it most helpful when someone asks me directly if one thing or another would be more helpful. It varies not only from person to person, but from day to day.
The less that is assumed about what would help me specifically, the better. I have had well meaning supporters and friends assume they knew what would help. I have appreciated most of it, even if it triggered me and/or wasn't helpful. I have tried hard to recognize the heart behind it. It's good to know there are people in the world who do care and who want to help encourage sufferers. Most of us have endured the darkest sides of humanity.
An encouraging text to one person may actually be a really emotionally triggering text for another. I once sent a fellow PTSD sufferer a text that would have helped me and it pissed them off. It happens. We both learned a lot. This isn't true for just PTSD sufferers but people in general.
Sometimes what I need more than encouragement is someone to "just" be there. Without fixing, or curing, or changing, but someone who gently reminds me they are there for me. Sometimes it is hard to read a text telling me there is a silver lining to every cloud when the rain is pouring down and it's so dark I can't see my hands in front of my face... and this happens. I find it so hard to respond to.
Sometimes what is really helpful to me is the friend who sends a text to say "This storm is awful. I'm here for you."
Sometimes, a message of validation and presence can do more to encourage me than a million encouraging texts ever could.