MuPDF

MuPDF is a lightweight and simplistic tool that lets you view the content of PDF files, without the need of Acrobat Reader. It can be seamlessly handled, even by inexperienced individuals.
This is a portable app, so installing MuPDF is not necessary. It means that you can store the tool on a USB flash drive or any other removable device, save it to any computer and directly run its executable file.
Therefore, you can always have MuPDF with you when you're on the go. What's more important is that the Windows Registry is not updated with new entries, and no leftover files can be found on the hard drive after program removal.
Upon program initialization, a file browser dialog pops up and you can use it to locate and open a PDF file. The interface of MuPDF is minimalistic, consisting of a standard window and without having any visible options.
But, if you are patient enough, you can discover some keyboard shortcuts to use. For example, you can use the dot, comma, 'Page Up', 'Page Down', left and right arrows keys to go to the previous or next page of the document.
MuPDF also integrates a search function that can be accessed by pressing the 'F1' or question mark button on your keyboard. However, the tool can only locate and highlight the first entry it finds. Other features of MuPDF include zooming in and out (the 'plus' and 'minus' signs) on your standard keyboard or numeric pad, switching to full screen mode (the 'F' key), rotating pages left and right by 15 degrees ('A' and 'S', respectively), as well as applying a negative or greyscale effect ('I' and 'C' respectively).
The application runs on a low-to-moderate amount of system resources, has a good response time and didn't freeze, crash or pop up error dialogs. Aside from the incomplete features provided by the search function and the fact that no help documentation shows the hotkeys, MuPDF offers first-time users a pretty straightforward solution for viewing PDF files.