When text in a document isn't created as PDF text, it doesn't get recognised as text. This affects the ability to highlight, search and other interactions with normal PDF Text.
For example, when you use our text box annotation in Drawboard PDF, because it's considered an annotation, it does not get saved as PDF text but Annotated Text. It is almost treated like an 'image' instead of text itself.
There's no way for Drawboard to insert PDF text that was not originally created as such(for now).
Another case this might happen: if you were to print this article from your browser used 'Microsoft Print to PDF'(which converts it to and XPS document first, then PDF) instead of 'Save as PDF'. You'd realise the whole document's text cannot be interacted with.
We're sorry PDF has to be this confusing, but we hope that helps explain some things!