Can you show us which code you have in which file?
Also keep in mind, you need to add the HTML code in the WebGUI under System-Settings ⇒ Invoices ⇒ PDF Footer.
As mentioned in my post, my CSS modification are in a special file which overrides the standard values of the file. Make sure you added your CSS code in the right file, assets/core/css/custom-pdf.css and you called the correct classes and ids from the additioned HTML code in the settings from before.
I would show you my code snippets, but they changed because I added a picture to my footer instead of real text. This may change on my side, but it is what I have at the moment. Nonetheless, I’m here to help you, at least trying to.
As said, show us the path and code you added then we can look further.
Also try out what @UnderDog recommended doing.
I suppose that I didn’t wait enough for the upload of one of the modified files.
Either ‘assets/core/css/custom-pdf.css’ or my proper invoice template file.
And I didn’t took notes of my way to edit the files.