You have two different problems here. The first is that not all PDFs show up on your Mac as searchable. The creator of the PDF has to make the PDF searchable, and if that doesn't happen, then you have to use OCR software to scan each page of the PDF to "find" the text and add it to the PDF. When you say that Spotlight doesn't find any PDFs when you search for "USB C" that means there is no searchable text inside the PDF. When PDFs are searchable, it means that, hidden in the PDF is the equivalent of a text file that is a layer underneath the image on each page. That's what Spotlight can find and index. If that text layer isn't there, there's nothing Spotlight can do for you.
Personally, I use PDFPenPro to OCR my textless PDFs. The Pro version has a batch command that lets you OCR hundreds of PDFs in a row. I also like that PDFPenPro simply modifies the existing PDF and adds the text to it. Some apps (especially in the Windows world) require you to save a copy of the exported PDF, which is a real pain. But there are plenty of other apps that do this, like ABBYY FineReader, DevonThink (I think the Pro version only). Evernote also does this, but it doesn't save the text to the file. Rather, it saves the text to its own database. So, if you ever take the PDF out of Evernote, it's still not searchable.
Once you get your documents OCRd, you're right to say Spotlight will only show you a list of documents with those words. It won't show you where the word is used in the document (your contextual paragraph request). That's where these other app suggestions come in, like Houdah, Yep, and again DevonThink. But you need to OCR your PDFs first.
