Apps or software! T don’t convert a pdf file or any base file with 100 % accuracy. Even, I do different document conversions, when we have mathematical or special symbols in base document, it will be impossible for a particular app or software to convert the particular formula or text as it is. So, companies require manual workers to do such jobs.
If you are pretty experienced in your field and have a lot of experience and clients. You may make very low hourly rates (3-7 USD per hour) so that's a no-brainer for most freelancers. Other freelancing sites may have the same idea, but some may charge more. If you can, try and test what works best for you and your level of experience as possible before you start doing the work. I guess another thing… that makes you money if you do it properly… is if you can make someone money. If you find that your work is not worth the time to do, and only a small amount of money is made at best… or you will never get paid, then it is time for you to take action. If you feel like your work doesn't really bring that much value to the market, then I guess we can.