When comparing KDE Okular vs (Spac)emacs with pdf-tools, the Slant community recommends KDE Okular for most people. In the question“What are the best PDF viewers for UNIX-like systems?” KDE Okular is ranked 1st while (Spac)emacs with pdf-tools is ranked 15th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Free and open source
Pro Trim margins
Easily trim margins either automatically or manually for easier reading
Pro Featureful
Pro Table selection
Pro Tabbed view option
Pro Supports touch interaction
Pro Extensible, customizable and scriptable
Being a Emacs plugin you can use elisp to customize, script and extend pdf-tools.
Pro Can treat multiple PDF's as one big PDF
Pro Nice search using Occur
Occur creates a list all lines matching a regexp or string in one or more pdfs and allows easy jumping between them. Really helpful when searching long documents like datasheets.
Pro Synctex support
Pro Easy installation
Although installing requires little more work than stand-alone readers, pdf-tools is very easily installed via Emacs 'list-packages' or even easier as a layer in Spacemacs
Pro Convenient default keyboard shortcuts
Uses emacs or vim-style navigation (via spacemacs/evil. Shortcut overview via transient state "SPC m .")
Pro Automatically exported notes
Notes can be exported automatically to and backlinked from an external org notes file using the org-noter or interleave package.
Cons
Con No XFA Adobe Forms support
Cannot fill PDF Forms created with Adobe.
Con Requires many KDE libraries
Con No middle-mouse auto-scrolling
Instead of scrolling automatically when holding down the mouse wheel and dragging, it instead zooms in or out, in contrast with many other programs.
Con Poor HiDPI support
You may have to tinker with QT__SCALE_FACTOR environment variables to get the desired size and not blurry content (this is a bug; see https://bugs.kde.org/show_bug.cgi?id=362856 and https://bugs.archlinux.org/task/54688)
Con No search results overview
You have to navigate to next/previous hit one by one
Con No "fit to page" option under printing settings
You can only print the content as given so, when you receive a bigger or smaller image thant the default of the printer, you will have to edit it first on other editor.
Con Slow scroll
Its new ultra-slow-scroll for PgUp and PgDn makes it unuseable.
Con Only for power users
Handy only for people that want good notes/annotation management. Otherwise using any other pdf-reader is recommended.
Con Too cumbersome for quick reading or annotating
Although setting up pdf-tools is not too much work. For quick reading using some default pdf-viewer like evince/okular/zathura is recommended. Also, except for the auto-export feature (with org-noter/interleave package), other editors like e.g. pdf-XChange Editor (via wine), Okular or mupdf have even more powerful annotation features.