That's the design behavior of Acrobat Reader on mobile devices.
You will have to select the highlight tool every time you want to highlight anything.
However this is not the issue on computers/laptops with Reader or Acrobat.
As you are going to purchase Acrobat Pro DC, so you will be using it on a system and you won't experience any such issue on your system.
[There is no Acrobat Pro DC for mobile device, you only get free Acrobat Reader for mobile devices]
On your Android device when you open a PDF file in Acrobat Reader DC you can tap and hold where you would like to highlight the text and use highlight option from the context menu. With this workflow you don't have to select Comment tool from the bottom and can work in Full Screen mode always.
Once a text is selected you will get the options of Copy, Highlight, Strikethrough, Underline. Check the screenshot below.
If no text is selected then then you will get the options of Note, Text, Freehand, Signature at tap & hold.
Let me know if this works for you.
Thanks. But no, the process you've outlined work for doing ONE highlight, but it does not keep the highlighter on so that one could read and highlight at will (just as one would do with a hard copy book and a highlighter)
Thank you for replying. Wow. I am dumbfounded that Adobe is missing the boat on this when it comes to Reader for Android. You would think that Adobe would realize that what people are looking for is the same experience as reading a paper book with a highlighter in hand - i.e., highlight at will once the highlight tool is on. Major miss. I was willing to pay for upgraded version of Reader/Pro if this has been the case. Not now. Big miss Adobe......
In the example provided above I never used the highlighter tool from the bottom list. I only tapped and hold on the screen to select the text and used highlight from there. If you are looking for a functionality where you just want to drag on screen and it select the text with highlight, that is not available as if now.