When Reading Mode is enabled, you can't copy text from a pdf as well as add annotation to the pdf. This behavior is by design.
You may switch to Single Page/ Continuous to copy text from a pdf.
Let us know if you have any questions on this.
Thanks for your answer! When I'm reading book in foreign language, I want to use Google "tap to translate" feature. But I can't use it in Reading mode. Is there some other way to translate text in reading mode?