There are two basic formats for PDF files.
Text based and image based.
Text based still preserves the letters, image based is like a photographic image — the text data is lost.
Text based lets you copy the text, image based does not.
Some not so good ways.
Print the PDF file and then scan it in and choose MS Word output format, OR Open a new word doc (for text based only).
Open the PDF file Highlight the topmost letter in the document.
Page down to the bottom and press
This is how to do it for an image based PDF: