It is a known issue that Max OS X PDF Preview does not support AcroForms properly, see this blog post that contains some details: Script to Fix Mac OSX Preview.app Form Fill and Save.
Among other errors, your PDF form fields have a missing piece of information: the appearance stream (a set of instructions that tell the viewer how the field value is supposed to be rendered when it is not being edited).
If you can specify which PDF viewer should be used for editing the forms, then avoid Mac OS X preview. If you need to support Mac OS X preview, then you can try to re-generate this information programmatically with any PDF library that allows filling out forms, or you can apply a form flattening process instead (converting the "dynamic" text into static) before exporting as jpeg.
Examples:
- If you have access to a Windows box and Adobe Acrobat, you can try with the script mentioned before.
- If you have access to a Windows box and purchasing a commercial library is an option, you can try with Amyuni PDF Creator (Disclaimer: I work for Amyuni Technologies). for regenerating the appearance stream you will need to enumerate the form fields, retrieve their values, set an empty value to them, then re-assign them the original values. For doing form flattening, you will have to set the annotation attribute of each form field to false.
- If you have access to a Linux box and a library with a GPL license is not an issue, you can try creating a Java application with iText, the method PdfStamper.setFormFlattening(boolean) seems to do what you need.