You will need to reboot the system. When your system restarts please hold down the Shift key and this will book you into the Grub menu.
Use the arrow keys to choose Advanced options for Ubuntu and select Enter
The next screen should list available kernels. Since this is 19.10 you should see a listing that includes:
Ubuntu, with Linux 5.3.0-18-generic
Select that one and press Enter and that should boot you into an older kernel.
These images below should give you a visual idea, ignore the versions:
Once you have booted into the system confirm the kernel level. An easy way is to open up the terminal and type: uname -a
Now try and open the pdf's and see what your results are and let us know.