It may not work for everyone but I can tell you what I do to solve this problem for myself. When I create titles for my book covers like to use outlines around the text and that will always cause the text to be rastered when I export it as a PDF. Before I export it I create a duplicate of the title and make sure that it is the top layer. Then I delete the outline from the duplicate top layer. On export the outline is good quality and the text on top is not rastered. When I export to PDF this way the text remains sharp and clear no matter how much I zoom in on it. I am not 100% sure but it seams to me that using any kind of adjustment layer or overlay on text has a negative affect on the quality of entire PDF so I try to avoid adjustment layers and overlays on text. I hope this helps.