Using mPDF without Composer
Using mPDF without Composer is discouraged and not officially supported.
Just use Composer
Generally, there is no real reason why you couldn’t use Composer. Install Composer locally, use it to install mPDF with its dependencies and use the result further along with generated Composer autoloading.
I do not want to use Composer anyway
Prepare to do a significant amount of work that Composer would have done for you.
1) Download the library and all dependencies
You can find library dependencies in the root composer.json file
2) Ensure all needed classes are loaded
This means both mPDF classes and dependencies classes.
You can do this manually (reload, find the file with missing class, add require call, repeat) or you can use an autoloading library of your choice.
Further usage is then the same as when using Composer,
just require your autoloading script instead of Composer