Make the file itself readable by the
_apt user. eg:
chmod 777 ./pdfsam-visual_2.1.4_amd64.deb
Have a look at the permissions of the file itself...
$ ls -l ./pdfsam-visual_2.1.4_amd64.deb
-r-------- 1 john john 105659960 Apr 4 11:57 ./pdfsam-visual_2.1.4_amd64.deb
Now that message again...
N: Download is performed unsandboxed as root as file
'pdfsam-visual_2.1.4_amd64.deb' couldn't be accessed
by user '_apt'. - pkgAcquire::Run (13: Permission denied)
The error is because user
_apt wants to access the local file. So you only need to grant access to this file. eg:
Now if you remove and install the error will be gone.
sudo apt remove pdfsam-visual
sudo apt install ./pdfsam-visual_2.1.4_amd64.deb