use big files plugin to open big files

That plugin is available through the Plugins Admin interface. Notice the caveat in the about-the-plugin box in the Plugins Admin (emphasis added)

Reads very large files. It does this by only showing a small buffer of the file and let the user move back anf forth in the page.
This is useful when you want to preview very large text files. Opening is done separately from Notepad++ and there is no option to save.
https://github.com/superolmo/BigFiles
Author: Claudio Olmi
Homepage: https://github.com/superolmo/BigFiles

Especially note: there is no option to save. So that plugin means the file is read-only, and cannot save edits made to the large files. If you want to save changes to the large files, that plugin will not help you.

В 32-битной версии процесс ограничивается лимитом в 4 ГБ, который опять же уменьшается вдвое, так как система резервирует 2 ГБ на процесс.
Но это не означает, что вы можете открывать 2GB файлы, потому что Scintilla выделяет информацию о стиле для каждого байта текста. По моим тестам это примерно 3x размер файла.

Как сказал Алан, 64-битная версия может открывать файлы большего размера.
Я смог открыть файл размером 1.8 ГБ без проблем, но работа с ним может быть затруднена/медлена.

With the 32bit version the process is limited to a 4GB limit which is halved again because the system reserves 2GB per process.
But this does not mean that you can open 2GB files, because Scintilla allocates style information for each byte of text. According to my tests this is about 3x file size.

As Alan said, the 64bit version can open larger files.
I was able to open a 1.8 GB file without any problems, but working with it may be difficult/slow.

Notepad++ v7.9.1 (32-bit)

You might try the 64-bit version of N++ as it is known to be able to handle larger files.

Thank you.

Не могли бы вы разместить здесь вывод отладочной информации?
Это в последнем меню, в ? меню.

Could you post the output of debug info here?
It is located in the last menu, the ? menu.