Thank you for posting your query here. Please help me with few details to understand your issue better.
Are you trying to edit the text in your PDF?
Application version?
OS version?
Do you have a premium subscription for Mobile Reader?
If you have a subscription try the below steps:
Add text
To add new text:
In edit mode, tap any blank space to show the menu.
Tap T+
Tap the desired text insertion location.
Enter any text.
Optionally, apply text formatting such as Bold, Italic, Underline.
Tap ✓ to save the changes.
Once you’ve exited the add text mode, you can tap the text again and select a tool from the menu to change the font style, size, weight, color, and alignment.