It is always important to verify whether the large file you downloaded is the file you expected to download or not. This is because files may change in some way from the original while downloading.
The first time I heard about hash tables was after taking a compilers course during my BSc. The truth is, I was not able to understand and appreciate their usefulness fully back then. Now that I know ...
In an ideal world, your PC should never be the only place where your important data lives. It can be one of the places, though, but the most critical files should be backed up following the 3-2-1 rule ...