If the file contains "real" text (ie, not a scanned image, but actual, selectable text) then the application will default to the Text Highlighting mode (you can change it to the Draw Mode by holding down Ctrl). If the file doesn't contain such text then only the Draw Mode will be available, as there's no real text that can be highlighted.
I am having the same issue - after a reinstall - it is a document which I have managed to successfully and normally highlight in the past.
Sorry for the delay in response.
Would it be possible if you can share the problematic pdf file with us? You may refer to the steps given in the following link on:
Also, let us know Acrobat dot version and the operating system installed on the machine. To check the version in Acrobat: Identify the product and its version for Acrobat and Reader DC
I'm having the same issues randomly several times over the last twelve months.This attached document is from an online pdf that I printed to pdf using the adobe printer function. First page highlighted fine. Second page had blobs.
I went back and directly downloaded the same pdf to my computer, and the highlighter worked fine in that scenario.
Here's my share link: Shared Files - Acrobat.com
Version : Adobe Acrobat Standard DC Version 2015.006.30418