I just had the same issue this morning, but in my case it happened with engineering drawings. It was obviously an issue with fonts, although I had no idea why these particular pages were affected and not others. In my case the drawings all were originally drawn in SolidWorks and all the text was Century Gothic. About 50% was gone. So I went to the first drawing, EDIT and selected the entire drawing. From the Format area on the right, at top, I picked Century Gothic. The entire page plus ALL THE OTHER PAGES came back to normal. I have never done this before and I cannot say it will last but at least I have full lines of text now.