I am also trying to add the html code inside the editor via source but the pdf file appears inside the preview but when I press save it does not appear and the code is deleted

Where the error_log file is constantly full of errors, sometimes up to 1 GB

If you are on a shared hosting plan I don't think you have any way to change the MySQL configuration. So, you can't fix the problem on a shared server. You would need a VPS hosting plan where you could make changes in the my.cnf file to change the MySQL server configuration. CHOOSE YOUR HOSTING PLAN WISELY! I would urge you to choose a managed VPS hosting plan. This will cost more (certainly much more than a shared server hosting plan) but the administration of a VPS server is much more complex than a shared server hosting and I think you would be totally at a loss dealing with setting up everything on a VPS server on your own - and we certainly couldn't help you with everything!

For the time being it MIGHT help (No promise!!!) to disable as much plugins as you can to reduce the number of simulatanious database connections. But if the max_connections value is very low it might not help even then. And if the connection_timeout value is too high (might be on a shared server) it might not even help with the lowest possible number of plugins in use.

I clearly see that I should upgrade because even after stopping a lot of add-ons, the problem still persists