Configure TOC in the PDF Documentation

Ivan, you can control this in the CSS file. Not sure if you have looked at pdf.css which comes with default new project. In the css file level 2 style should be like the following

p.pdf-toc-level2 {
display: none;
}

You will have to do it for all levels till 6 if you want just heading level 1 only.

This can be done in the CSS editor Author mode also. Go to Paragraph Styles > p.pdf-toc-level2 > LAYOUT > Display in the dropdown select "none".