Info, driver, etc for HP LaserJet Pro MFP M26a are here.
After you have downloaded the sw package, unzip it.
Open a Linux terminal, go in the directory where you unzip the sw package (cd xxxx) and enter in super user mode (sudo su).
In the directory you must see the file/directory below.
Now change the permissions of the file:
hplip-3.18.6.run
for do this use the command:
sudo su (and enter the password)
chmod +x hplip-3.18.6.run
Now run it using the command:
./hplip-3.18.6.run
and follow the display indication.
At the end of the installation reboot your Ubuntu PC.
That is all.