Writing on to any PDF file is not possible unless the original file creator allows writing on it (forms and such stuff). However, there are various software's available that allow you to do write on them even without the permission. Unfortunately all Windows-based software's are being sold commercially and not available for free use. I often have to write in PDF files and I refuse to pay anything to do so.
Foxit Reader is PDF reader whose reader feature is available for free use, ie, you can read any PDF files using Foxit for free. It also has a editor feature in the software itself called Typewriter. However for unrestricted use of the software you need to purchase a license from the developers. If you try to edit and save a PDF file using the free version, it will add an ugly watermark to the saved document.
However even the free version allows you to take a printout of the modified document without any watermarks added in it as long as you don't save the changes.
PDF reDirect [free version] is a software that is used to create PDF files from any document whose printout can be taken. Once installed, PDF reDirect appears as a virtual printer in your list of installed printers.
So next time you make any changes to the PDF file using Foxit Reader, instead of saving it, take a printout of it selecting PDF reDirect as the printer from the list. Foxit is foxed into thinking that you are taking a printout on a sheet of paper and so does not add any watermark, whereas you are creating a brand new PDF file with all the changes and editing you did using Foxit.
In fact you can use all of Foxit Reader's paid features in the free version in this way - just create a new PDF file containing the changes made in Foxit.
If you are using Linux, you may try out PDFedit for the same.
If you wish to rearrange the pages or delete pages in a PDf file, then you may try out a free online service - PDF Hammer to do so.
Do let me know if you have any doubts, queries, suggestions or criticisms about this post.