Have you ever wanted to add an image, a signature, delete or add a page to a pdf file? You can do that with software such as Adobe Acrobat DC ($12.99/month); PDF Pen (mac $74.99) and PDF Expert (mac $59.99), all great software if you do a lot of pdf editing. However, if you only need to occasionally modify a pdf document, there is a free open source solution available to you.
LibreOffice 6 is an open source competitor to the Microsoft Office Suite. It contains the traditional features of a word processor, spreadsheet and presentation module (Word, Excel and PowerPoint functions) and an Access-like database. In addition it has DRAW for creating vector graphics and other artwork and MATH for editing mathematical formulas. LibreOffice is available in versions for Windows, MacOS and some forms of Linux. Best of all, it is FREE! You can learn more about LibreOffice 6 and download it here.
The DRAW module allows you to create a pdf from scratch or edit an existing document. Once you have opened the document in DRAW, you can add, edit or delete text; add or delete pages; add images; add or change tables; add a digital signature; sign with a watermark; or set encryption/permissions, to name a few things you can do. When you are done with your changes, you just export your masterpiece as pdf.
If your documents contain complex formatting, LibreOffice is probably not going to be your best solution, and you will want to use one of the more fully-featured tools. However, if you only need to revise pdfs occasionally and have fairly basic editing needs, check out LibreOffice as free is a very good price! Learn more about editing pdfs using LibreOffice here.