PDF documents are great but there may come a time when you are reading/reviewing one when you need to roughly know what the word count of the document is. By their nature PDF’s are usually full of charts, images and tables that make the task difficult. Most (all?) free PDF viewers don’t give the reader a word count in situ. So you have to use either expensive software or another method.
The simplest and easiest way ( and also free ) I know is to simply copy and paste the contents of the PDF document into a word processor (I use LibreOffice but Word or whatever would do).
Open the PDF document in your PDF viewer (I use Evince on Ubuntu 18.04) Then simply do a ctrl-A, ctrl-C
This copies all the text from the beginning to the end in the PDF. But not images or tables.
Then open a new LibreOffice document and ctrl-V to paste the text into it. The formatting will be all over the place. But that is not the reason you are doing this. You don’t care about the formatting. It is irrelevent. You are simply doing this to get the word count.
Goto menu item “Tools” then open word count . This will display both the word count and the character count that you have just pasted into Libre Office. When you have the information you want just cancel the document. It has no other purpose.
It is a bit rough and ready and will include a few bits and pieces you wouldn’t normally count. It will also exclude any text that may be presented either as an image or as part of an image. But for most purposes it is good enough to give a valid approximation the the PDF document word count.
Hope that helps.