PDF files never start off as a PDF file, they are digital print outs of another file created in a program such as Word, Excel or a graphic design program. There are expensive programs available to edit PDFs, but Word does a good job for basic edits.
The best way to edit a PDF is not to edit it at all, but make changes to the original file the PDF was created from, and then create a new PDF file once the changes are made. If the PDF file is all you have, it can be edited using Word with reasonable results, without having to pay for expensive professional software for this purpose.
Click on the File menu in Word, select the Open menu and browse to the folder that contains the PDF file. Once you have located the PDF file, double click it to open it. Word will display a message warning that the process to convert the file into Word could take a while. How long depends on the complexity and size of the file.
What you have now is a copy of your PDF file in Word format so any changes you make to this file will not affect your original PDF file. The first step should be to save your file in Word format so any changes you make are saved. Press the F12 button to open the Save As dialog box and save the file on your computer.
Not all PDFs will open up perfectly, and it make take some work to adjust and maybe redo parts of the file, as well as the changes you want to make to the file itself. If the PDF contains text in an image, you will not be able to edit that text. Another issue you may find is that the font type in the PDF will not match the fonts in Word.
Once you have completed the edits, save the Word file and then save it a second time but this time choosing PDF as the file type to save the file back into PDF format. If you ever need to make changes to the same file again, go back to the Word version of it rather than trying to convert the PDF file back to word again.