Take a look at this article: https://bitmiracle.com/blog/ocr-pdf-in-net
Basically, you need 2 tools:
PDF library to convert PDF pages to images. There are many paid .NET libraries for that. If you are looking for free tools - look at C/C++ libraries like Ghostscript, xpdf, muPDF.
OCR engine to recognize text on images. Tesseract (free and open source) is ...
Syncfusion Essential PDF supports OCR by using the Tesseract open-source engine. With a few lines of code, a scanned paper document containing raster images is converted to a searchable and selectable document.
You can able to get the data from the invoice PDF or image using OCR processor in our Essential PDF. Please refer the below link for more details,