SUMMARY
What if we had the ability to subdivide an Ebook

STEPS TO REPRODUCE
1. Open any Ebook
2. Read Ebook to somewhere like chapter 3
3. Realise that you've forgotten the content of chapter 1

OBSERVED RESULT
You get frustrated that you have to reread the book from the first chapter and 
that the bookmark feature is doing no justice

EXPECTED RESULT
You have the ability to open the book in parts where each part is it's own 
book (subBook) and each subBook has it's own bookmarks, history and other stuff 
that comes standard with each book , only that it's a cut out of a bigger book.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 0.24.0
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 4.8.7

ADDITIONAL INFORMATION
Okay , I admit that this is not a bug report at least not that of Okular , but rather that of my brain ...

COMMAND_LINE : brain --wakeup --do_stuff
VERSION      : 1.3.8
OS           : HumanAnatomy
ARCH         : prim_86e10n
BUG          : poor memory capacity

First off let me point out that I'm having doubts that Okular ever had any bugs
(sure I've been using it for about six months now , but still) it seems to work
just fine. Here's the issue , I've been reading this ebook 
(Precalculus-OP_88kwxYo.pdf  from Openstax) which is rather enormous (1156 pages) and somewhat interesting ,and for the third time now I keep rereading it 
from chapter one everytime I get to chapter three not because I want to but because I keep forgetting the contents of previous chapters , sure I could take notes but (besides saving the environment by not making use of paper [for writing]) I'd rather just read the original content or make use of the computer to take down notes but the switch from Okular to emacs is somewhat unsavoury especially having to do it every few seconds (one might as well rewrite the book) , in summary I'm not good at taking notes. So I came up with the idea of reading the book in parallel (as opposed to the way ebooks are read). Enter the 
Okular exclusive SubdivideBook (or whatever the developers choose to name it).

Normal one opens an ebook and Okular conveniently takes them to the last page they've read , great so what's the problem ? After going back to the first chapter to look up a section of interest (from the third chapter) I normally would exit Okular there and then (given that the section of interest is in fact
the entire chapter ), so when I reopen the book I have to whizz through hundreds of pages just to get to the third chapter and if I've forgotten the section I was in reread the entire chapter from the bottom. With the subBook feature I could open the book , choose a subBook that  with chapter one read it (of course) and when I reopen the book , choose a subBook with chapter three , sure 
I could use the navigation options provided by Okular but then again reopening the book would force me to reread a chapter (or some subsection thereof) thus destroying the whole purpose. On the other hand each subBook has it's own last_page_read (or whatever it's called in the sources).

A possible implementation would be a "Make SubBook mode" , in this mode there are (at least) four actions that can be performed, ADD_PAGE , REMOVE_PAGE , BUILD_SUBBOOK , CANCEL. The "Make SubBook" function could automatically add the current page (page being read on the call to make a subBook) and thus performing the ADD_PAGE function , on the call for the next page (which would now be the current page) ADD_PAGE could be performed again , the call for the previous page could result in REMOVE_PAGE being performed unless the the previous page is being requested from the first page of the subBook in which case the first page would be replaced by the current page on every previous page request. After the subBook has been completed BUILD_SUBBOOK could be performed. It's important to realise that a subBook is not a complete book in it's own right but rather a slice of a bigger book (or optionally another subBook [recursive subBooking]).
Just in case it's not clear : ADD_PAGE      --> Add a page to the subBook stack
                              REMOVE_PAGE   --> Removes a page from the top of the stack
                              BUILD_SUBBOOK --> Creates a subBook from the subBook stack
                              CANCEL        --> Cancels the entire operation

Please implement this feature if convenient , if inconvenient implement it all the same.