Bug 1827797: PDF viewer not hiding submenus on loading

This is due to the PDFjs using it's own .hidden css
and we changed 'hidden' class to 'd-none' in the html.
We need to change it back to 'hidden' again.

behatnotneeded

Change-Id: I91fd9dbd890d1e0effa98f667461fc49b24dc154
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>