Here's how to save a whole web page as one PDF with Kortex, then add it to Gemini Notebook, formerly NotebookLM. Open the article in a normal browser tab. Wait until it finishes loading, so all the images and text show.
Click the Kortex icon in the toolbar. The side panel opens beside the page. Its top card is about the tab you're on.
Click Capture as PDF. Kortex scrolls the page from the top, hides sticky bars and builds the PDF. Leave the tab open until it finishes.
Don't use Import page for this. It adds the page to a notebook, and makes no file. The PDF is a file on your computer.
If you can't find it, check your browser's downloads. Then open the notebook in Gemini Notebook, choose to add a source, and upload the PDF. Kortex isn't part of this step.
Find the full steps and fixes on the Kortex website.