unable to highlight a pdf with ipencil - ipad pro

Hi

I was working on a pdf, highlighting it. 
Everything was heaven, but suddenly the highlighting and the underlining tools stoped to work. Instead, when I touch the screen with the pencil while one of these tools is activated, I get a loupe tool.
I guess I  pressed a key, but couldn't find which one. 
Can anyone help me?

In advance, thanks a lot!!

JK