Change min-width to 100%

Hi, in the css for the plugin this is stated:

 #mainContainer {
    min-width: 350px;
}

I can’t seem to target that class, so how can I change it to

 #mainContainer {
    min-width: 100%;
}

I’m asking because I have pdf:s in columns, and when they get below 350px, the pdf expands out of the column = not nice.

The page I need help with: [log in to see the link]