Startup Idea: Tabletop Game Rules Compilation App
There is a rising market for things related to tabletop games like board and card games. There are thousands of games out there, each with an instruction manual. Usually these manuals are available from the publisher website as a pdf.
An app that could compile the rules for a broad range of games in one easy to access place could potentially draw in a large portion of this growing market as people try these different games with more and more complexity of their rules.
There are plenty of online retailers for relevant ads to target the app's audience and a premium version could maybe have some sort of bonus features such as keeping up with new kickstarters or something that are board games, card games, tabletop rpg products, etc. Yes, downloading pdf files from individual websites as needed. Boardgamegeek.com and reddit.com/r/boardgames are good references as well for specific case rule verification.