All of my font is faded when adding a comment. Please help.

sry, don't have answers to help. just here to say the PDF is literally one of the worse file types I've ever used.

Hopefully you've found a fix for this issue since posting. I'm running Adobe Acrobat Reader DC and an update in the last month has started doing this to me too, it worked perfectly before. It doesn't give the option to set opacity on comments. Here's my work around: Put the text in a text box (set background & boarders to white) instead of a comment. Text boxes you can set the opacity and if it's at 100% won't fade out at all. This won't work if you're trying to get close to other text and your white backgroud box starts covering the document text, but it's something. 

Any other suggestions? Is there a way to reset all of the default settings in Adobe Acrobat Reader DC? I've already tried uninstalling and reinstalling. I'm using a Mac.

It sounds like the default transparency setting for your comments to reduced. Is it something like this?

Also, what type of comment are you using? Highlighter tool? Sticky notes? Text markup tool? Drawing tools? Please elaborate.