PDF files

Adobe's popular PDF format is a wonderful boon for reading documents they way they were created on any computer. But the free Adobe Reader program is bloated and "phones home" regularly for updates.It also doesn't let you point out errors or add comments. What can do the latter is Tracker Software's free PDF-Xchange Viewer. This application can't create PDF files, but it supports many kinds of markup, from highlighting to circles to sticky notes--and you can even export the product as another PDF. The download file is 10mb, which is a bit hefty, but still much smaller than Reader.
Another useful PDF reader is Foxit Reader, even smaller than X-Change Viewer (about 4mb download). As with many free programs, there's some advertising to sit through. The install also asks to add a Foxit toolbar as well as an eBay shortcut, and it wants to set your homepage to Foxit Software. I don't recommend any of these, so watch carefully. Also, though the free version lets you alter content, there are watermarks in the finished product that are only removed when you purchase the $40 Pro Pak version.
For the smallest of the small, though, Sumatra PDF wins the prize. It's only 1mb in size, which these days is tiny. It has a very basic interface, and really only does one thing: reads PDF files. But it is free.