It depends on whether your PDFs are machine readable or not. Are they scanned images as if they were faxed or scanned in through a scanner? Or is it possible to open the PDF and select the text and copy it out? If it's possible to select the text with your mouse cursor and copy, then you have a lot more options.
If your PDFs are images, then I would say for the most part the options will not be free. Some Some examples here would be AWS Textract, Azure Forms Recognizer, and Vidado (for handwriting and unclear text).
If your PDFs are like a normal PDF where a Word doc was converted to a PDF or someone originally created a form as a PDF, then PDFSharp, iTextSharp, etc. are free options. There are some assets on the Digital Exchange, but I haven't looked at them in months, so I hesitate to suggest a specific one. At my organization, we use a combination of PDFSharp and some other utilities as needed.
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Dave Morris
Cano Ai
Atlanta, GA
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Dave Morris, 3Ci at Southern Company