Text in our PDF export should generally be selectable and copyable. I just tested one presentation template I recently used, and it is indeed so.
I know there were bugs forcing text to be rasterized in certain cases, mostly related to text effects. To my knowledge, they should have been worked around by adding a layer of transparent text on top.
If you know any design that produces PDF file where text becomes not selectable, that might be a bug, and it would be appreciated if you can provide more details so that we can investigate and possibly fix.