■ Annotate PDF books using Sumatra PDF reader

There are two kinds of people in the world, those that prefer traditional printed books, and those that go for electronic versions thereof (e-books, PDF documents etc). When it comes to timewasting books like novels you read on holidays, e-books are preferable. But what about academic books? Electronic versions of scientific or computer books are convenient (you can easily search in them), but you still miss out underlining and highlighting paragraphs, scribbling explanations, annotations and other such hard-copy operations that help you revise a book.

There are many PDF reader tools that add text highlighting capabilities, e.g. Adobe Reader DC (even Edge browser can do it) but as we've seen Adobe reader is rubbish in terms of shell integration and working with xplorer², plus it is slow and resource hungry. My favorite PDF reader of late is Sumatra PDF, a lightweight PDF and ebook reader that doesn't even need installation. The other day I read about a hidden text "annotation" feature available in a prerelease (test) version, and it works nicely to highlight PDF document text as such:

annotate PDF book

Here are step-by-step instructions to highlight PDFs with SumatraPDF:

Download the prerelease version. Not sure why this feature is stuck in test mode for a couple of years now, but one day it may find its way to the stable version.
2022 AD: If you get the latest stable version 3.4 you will see that the annotation feature is finally fully supported. On top you can even underline and add text comments to PDFs!
As you read the PDF book, use your mouse to select the text to be highlighted. Press A key (for Annotate) to flip highlighting on/off.
To keep the highlights you must Save the PDF document pressing <CTRL+S> keys. I prefer to store the marked text in a separate file without touching the original PDF book. You need to tweak Sumatra PDF's advanced options to set SaveIntoDocument=false (more details here), which will create a sidecar file xxx.PDF.SMX next to the original PDF document.

If you want more text highlighting capabilities, someone has created an AHK script called Sumatra highlight helper that promises to be more stable (especially in cancelling highlights) and offer more tricks like finding the next highlighted text in a book.

I still miss a PDF reader that will allow me to show 2 different parts of the same book with a splitter (page flipping), but text highlighting is a big step forward reading all those technical books on artificial intelligence. Thanks Sumatra PDF!