Try to download the .pdf file again as there may be been some data corruption either in the download process or perhaps in the file itself. If it still fails, then if possible try to download the same file using another device, e.g. a pc etc and see if it still says that there is damage. If so then the file itself it faulty.
With regards to the upgrade message. That is to advise you that there is an updated version of the OS available for you to install in the tablet.
You do not have to do this unless you want to. It may be prudent to get more familiar with your tablet before attempting to upgrade the OS. It is normally a safe procedure but there is always the possibility of a problem.
In the future when you are more familiar with how your tablet operates you can always upgrade then. It may be also that you will be forced to upgrade, if a newly downloaded app doesn't function properly or if you desire a new feature that has become available due to the upgraded OS.
If you do wish to upgrade the OS be sure that before you do that you have made a backup of your tablet using the tablet's backup feature. That way you will have a copy of your data and downloaded apps etc in case the upgrade fails for whatever reason and you have to reset your tablet.