Free version of PDF-XChange Editor (Windows, Linux via Wine) highlights all entries for the keyword, and on top of that adds colored ticks on the scrollbar à la Chromium.
Also, navigation (scrolling/jumping back and forth across pages) is as fast and the program remains as responsive as simpler yet more lightweight alternatives such as MuPDF or Sumatra PDF even when working with huge PDF files (hundreds of megabytes, thousands pages with illustrations).
All instances of a given word can be displayed by using Find… (Ctrl + F) tool for a quick preliminary lookup:

Using Search… (Ctrl + Shift + F) tool for additional gathering of a complete list for all encounters.
Here, it only took one second to find 1139 entries in a 1300-page document, which I think is quite fast. Note, however, that this is "clean" document properly compiled from LaTeX source, and on a file that's been scanned and OCRed it could take longer:

Finally, by choosing Options… → Annotate search results → Highlight Search Results one can add permanent marks to the document and also save the changes to the PDF file:

Disclaimer: I am only a user, not a developer of PDF-XChange Editor and I'm not affiliated with Tracker software company.
It had to be verified whether it does actually work in Acrobat Reader DC (or XI), but it is possible to parse the document page by page, and search for the according keyword, and create the highlight annotation, and save the document. – Max Wyss – 2015-07-19T20:14:22.420