Sometimes a PDF is "locked", by the document's creator, so you cannot edit it.
If it will output to a printer, tere may be a way to circumvebt the lock.
It is possible to install a PDF "virtual printer", which is designed to create a PDF file from word processed documents or artwork. You can print your PDF via this to create a new PDF file which is not locked.
The process is not foolproof. Sometimes the new file will read the original text and re-write it as text you can copy. On other documents, the new file will treat the original text as a graphic, so you will not end up with text to copy.
As has been previously suggested, unless the original document was extremely large, it would have been quicker to read the document and re-type the text into a word processor or text editor than wait for our suggestions.
Quack!
GtD.