What you need to do is take advantage of the /F key in the FDF Syntax - it is a link to a specific PDF document, so then all you need to do is stream the FDF to the browser. It will open the FDF in Acrobat/Reader (assuming the user hasn't played with their application/vnd.fdf MIME type) which will then see the /F key with the PDF URL, open that PDF and merge the FDF data automatically.
Check out the FDF Toolkit, available here:
FDF is somewhat cumbersome to use as it doesn't look like anything that most people are familiar with unless you've spent time working at Adobe. However, XFDF (the XML expression of FDF) works equally well for form data and is fairly straightforward to use. Rather than send the PDF to the browser, you send the XFDF with a mimetype of application/vnd.xfdf and Reader (or Acrobat) will open the file and then fetch the PDF that's referred to in the "f" element. Essentially, you don't add the data to the PDF, you add the PDF to the data. Below is an example of a simple XFDF file.
<?xml version="1.0" encoding="UTF-8"?>
<xfdf xmlns="http://ns.adobe.com/xfdf/" xml:space="preserve">
<f href="DemoForm.pdf" />
This really helped. I was able to figure it out. Thanks Joel!