There is no exact number of PDFs that guarantees 20 will be indexed, but many people upload around 25–30 good quality PDFs to reach that number. The niche does not strongly affect indexing speed, because low competition helps with ranking, not indexing; what matters more is that the PDF has real text, is easy for Google to crawl, and is linked somewhere. The safest and most reliable way to index PDFs is using Google Search Console, while a popular paid option many people use is Rapid URL Indexer, though no service works 100% of the time. PDF indexing can take a few days to a few weeks, but with Search Console it often takes 1–7 days, and with paid tools it usually happens within 2–14 days.
