It is possible. Depending on the type of changes you want to do to the PDF your might not be able to collect your responses via FormsCentral though.
If you want to create a PDF in FormsCentral without a submit button you can do that and do any changes you want to the PDF using Acrobat X.
If you want to collect responses in FormsCentral then you will have to tell me what changes you want to do to the PDF and I can tell you your PDF will still be compatible with FormsCentral.
Before you can modify the PDF in Acrobat X you will need to save the the forms without the Reader Extended rights. To do so Open the form in Acrobat X and do File > Save a Copy...
You can then post the PDF on your website.
Hope this helps