split PDF by bookmark level of choice
At present it is only possible to split a PDF by top level bookmark; it would be great to have the option to choose the bookmark level (e.g., if there are nested bookmarks in a document). There are PDF editors around that do have this option.

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VIVEK CHOUDHARY commented
Any users can easily divide PDF into multiple PDFs free download without facing any technical error. For this, you can download PDF Split Tool on your system. It works on all Windows OS editions i.e. 8, 10, 11, Vista, XP, etc. and Mac OS versions like High Sierra, Big Sur, Catalina, Mojave and many more. This software helps to maintain PDF attributes while dividing the PDF files into various PDF files. Also permits all users to enhance the technical skills and test its advanced functions with its free trial version. It provides the facility to save the resultant output in the desired location path. There is no need to install the Adobe Reader for splitting the PDF files. Its demo version allows splitting first 5 PDF files/folder with the watermark for free.
Read More: https://www.cubexsoft.com/pdf-split/