DOC: OCR vs PDF text extraction

Closes #1073

docs/user/extract-text.md

@@ -58,3 +58,45 @@ the way PDF stores information just makes it hard to achieve that:
 
 And finally there are issues that PyPDF2 will deal with. If you find such a
 text extraction bug, please share the PDF with us so we can work on it!
+
+## OCR vs Text Extraction
+
+Optical Character Recognition (OCR) is the process of extracting text from
+images. Software which does this is called *OCR software*. The
+[tesseract OCR engine](https://github.com/tesseract-ocr/tesseract) is the
+most commonly known Open Source OCR software.
+
+PyPDF2 is **not** OCR software.
+
+### Digitally-born vs Scanned PDF files
+
+PDF documents can contain images and text. PDF files don't store text in a
+semantically meaningful way, but in a way that makes it easy to show the
+text on screen or print it. For this reason text extraction from PDFs is hard.
+
+If you scan a document, the resulting PDF typically shows the image of the scan.
+Scanners then also run OCR software and put the recognized text in the background
+of the image. This result of the scanners OCR software can be extracted by
+PyPDF2. However, in such cases it's recommended to directly use OCR software as
+errors can accumulate: The OCR software is not perfect in recognizing the text.
+Then it stores the text in a format that is not meant for text extraction and
+PyPDF2 might make mistakes parsing that.
+
+
+### Can we just always use OCR?
+
+You might now wonder if it makes sense to just always use OCR software. If the
+PDF file is digitally-born, you can just render it to an image.
+
+I would recommend not to do that.
+
+Text extraction software like PyPDF2 can use more information from the
+PDF than just the image. It can know about fonts, encodings, typical character
+distances and similar topics.
+
+That means PyPDF2 has a clear advantage when it
+comes to characters which are easy to confuse such as `oO0ö`.
+**PyPDF2 will never confuse characters**. It just reads what is in the file.
+
+PyPDF2 also has an edge when it comes to characters which are rare, e.g.
+🤰. OCR software will not be able to recognize smileys correctly.