Please give much more detail on the steps you took to open the file. 

1. Did you use your Ipad to download the website with Httrack? Or did you
download the site on another PC and copy the site to the ipad? 

Windows example: If I download mywebsite.com, then Httrack will create a
directory called mywebsite.com\, and under that dir is
mywebsite.com\index.html. Index.html is the file you would open first.
Sometimes it's called index.htm, index.asp, or a different extension. But look
for a file under mywebsite.com\ that is called "index".