Hey there lys-milena,
We understand when you insert a blank page into a PDF with the Preview app, those changes aren't being saved. We'll be happy to help.
To begin, take a few moments to double-check the steps outlined here: Add, delete, or move PDF pages in Preview on Mac
This explains how you can modify PDF pages with Preview.
Afterwards, let's try duplicating the PDF and saving a copy of it somewhere else or with a new name to see if the same thing happens.
Let's also try testing with different PDF pages to ensure it's not just an issue with one.
If the same thing happens, we'd like to get you in touch with the Apple experts using this link: Get Support
Let us know if we can help with anything else. Cheers!