I have found a way to view the hidden text on my Chromebook. It turns out that the Chrome brower native viewer works.i.e. On my Chromebook I can open the PDF file in the Chrome browser and the hidden text is revealed when I mouseover the area on the left side of the page.Note that this doesn't work in the Chrome browers "preview" feature. i.e. So you can get it to work by right clicking the pdf file in Files, and opting to view it with "View".