Essentially what I want to do is construct a web-app that would accept a file containing list of vocabulary words, likely numbering in the hundreds or thousands, then return web article results ordered by the percentage of each page's contents that belong in the given vocabulary list.
In this case, the user would never directly interact with the search, only provide the list of keywords, so I'm looking for an API I can implement into the background of the app. However, I haven't been able to find any search engine tools that would provide me with what I need. Taking Google's Programmable Search Engine as an example, there is a hard, relatively low limit on the number of keywords/characters that you are allowed to search with. I suspect that there's no search engines which would provide the percentage-of-keywords feature, and that I would have to crawl each page returned manually to calculate such a number, but being able to input all of the keywords provided is a must. This would be a non-commercial project, so free or open-source options would also be ideal.