How can I split a large book pdf in to separate files of different chapters automatically

​@resourceful_mind4ac1 I ahd to Google this, there is an free open source app called PDFsam (PDF Split and Merge) which splits, merges, extracts, rotates and mix PDF files. I sounds simple enough, Select Split and choose Split by bookmarks. There is probably more to that?! 

I would duplicate the large eBooks just in case though before using this free open source app! 

Most programs capable of creating a large book can create bookmarks, including Adobe InDesign, Adobe FrameMaker, Microsoft Word, Affinity Publisher, QuarkXPress, etc. 

Create the bookmarks for the chapter paragraph style (you DID use paragraph styles, right?) and Acrobat can split the PDF under the Organize Pages tool (Split option).

You cannot do this fully automatically. If there is anything “obvious” on the pages, and you can detect this feature with JavaScript, you can write a script to find chapter pages, and then create the chapters that way. The “obvious” thing could be certain text that can be extracted and recognized like “Chapter”, or it could be that the first text element always starts about a quarter of a page down from the top whereas every other page has normal margins. This all would require some JavaScript programming.