How to clear the contents under /run/user/1000/doc?

I am on Ubuntu 20.04 and installed evince via flatpak, when I open evince, it usually makes a copy of the opened file into /run/user/1000/doc. I learned that /run/user/1000/doc is the temporary directory for storing file used by running applications, but when I quit evince the files created by it does not go away. Even if I reboot my machine, the files still exist. As the directory cannot be accessed in the usual way, it is not writable and cannot be changed with chmod. How can I remove the contents of /run/user/1000/doc ?