My highlight tool stopped working suddenly

Hello there , I've been working on this huge file on my ipad for the last month without any problems in highlighting any text . 

suddenly last night the highlight tool and the underlining tool stopped working .

actobat would not recognize any text to highlight even the previously highlighted ones .

I tried uninstalling and reinstalling.

i tried restarting my ipad several times .

i tried updating the app , and even updating my ipad .

nothing seems to be working . 
what's more interesting to note is that it also stopped working on my phone (ios) at the same time , but the highlight tool is still working on my PC (windows) .

im using my PC for the time being but it's much more convenient to edit on my ipad .

is this an ios version problem ? How can i fix it ?