1. the term "memory" is ambiguous. it can refer to RAM or the storage space available on an SD card, hard drive, thumb drive, or other type of flash memory, like the built-in space on the internet tablet where the operating system and programs are installed by default. if you can successfully download a pdf but get a memory error when trying to OPEN it, that error message is referring to RAM, which the internet tablets are somewhat short on, but you can use up to 128 MB of one of your memory cards to act like RAM (this is called virtual memory) and you basically have 256 MB to work with (which seems like it should be able to handle most PDFs...). you could try enabling or increasing your virtual memory (in control panel) if you haven't already, or give up and read the pdf on a normal pc, which probably has a**loads more RAM than an internet tablet.

2. You can install applications on memory cards, but you have to move the whole operating system to the memory card, which is too much trouble for me. Maybe some day, if i get really bored. 2 methods below.
http://www.internettablettalk.com/fo...ead.php?t=8631
http://www.internettablettalk.com/fo...d.php?t=19639&

3. there are no viruses on internet tablets AFAIK. generally, there aren't many viruses out there for linux--one of the many advantages of linux.
http://en.wikipedia.org/wiki/List_of...mputer_viruses