Depending on the PDF file, you could try exporting it to Microsoft Office, making the changes, and then converting the document back to PDF. However, this will require you to have both Adobe Acrobat X and Microsoft Office 2008 or higher. That setup had worked for me once or twice, you might want to see if you can use LibreOffice or similar open-source word processor, but I'd expect worse results than with Microsoft's product.
Adobe Acrobat X allows you to comment text or pictures within the PDF file. In case this is what you've meant by writing in it.
There is a list of open source apps that you could try using to edit your PDF file. This is a free of charge alternative, but the results are often unpredictable. Check out the list here: List of PDF Software.
