How to copy text from a PDF
Use PDF to text to get everything in a .txt file. If you can't select words in the PDF, it's a scan and needs OCR.
Why copy and paste goes wrong
PDFs store letters by position, not as sentences, so selecting across lines or columns can jumble the order. Extracting the whole file is usually cleaner.
Tidying the result
Line breaks follow the page. To join a paragraph, paste it into a word processor and replace the line breaks.