>Any advice how can I fill in fields in PDF document from vfp app?

depends... for a dev PC Acrobat can be automated, if you have a license. If you want to deploy to many PCs and have many fields, create fdf file to fill in form fields in Acrobat Reader and you stay close to Adobe. There are probably tools in between, but not from Adobe itself - unless you want to populate via embedded Javascript and putting parameters on URL line or later loadable data script.

http://blogs.adobe.com/pdfdevjunkie/2009/12/populating_pdf_form_fields_fro.html
or
https://forums.adobe.com/thread/741050?tstart=0

Other options are to create the whole PDF each time as a report driven from a cursor or utilizing some other tool set via COM, for instance Python libs, or calling up WebBrowser plus specific Javascript.