Sorry to disagree but I have an order form on my iPad 2 for our companies web page I created 3 or 4 years ago with Adobe. Adobe Reader allows me to enter form data and saves it. It has text entry areas, drop down lists and numeric fields.
To the OP: Depending on how the PDF is sent to you use the "Open In" function to open it in the Adobe Reader instead of iBooks.
Drifting slightly askew of the topic:
There is a difference between Dynamic PDF (also called Rich Media PDF) and editable PDF Forms (text boxes that you can type in). The first allows things like videos and interactive media elements. It requires embedded Flash. Which is why it won't work on the iPad. The second is limited to a few specific field types like text, dates, check boxes, radio switches, and such.
Here is more information than you could possibly want to know about the various flavors of PDF.
http://en.wikipedia.org/wiki/Portable_Document_Format#Interactive_elements
There are several apps that will let you fill out PDF forms, the ones with active text boxes and such. Adobe's PDF reader is the free, and basic option. PDF Expert by Readdle is another, and existed before Adobe got around to releasing their own app on the iPad.
Just a few weeks ago Reeddle released their newest app; PDF Office. You can actually create editable PDF forms using this app. The form creation part is subscription based, but you could easily subscribe for the month or so it takes to create your forms, then cancel until the next time you need it. You'll still be able to read, fill out, and annotate.
Prior PDF Expert uses were grandfathered with a years subscription, so I've been playing around with it for fun.
Back on topic:
Yes, the best option if you need to print, edit, annotate, or fill out PDF files or forms is to use a PDF app made for the task. Adobe Reader is a good free solution. For more options PDF Expert, GoodReader, and PDF Office are the ones I use. Documents by Reeddle (they do a lot of PDF apps) is also good, if you're looking for a more general file reading solution that has decent PDF features.
Right now, as you may have guessed, PDF Office is my favorite to play with for forms. PDF Expert is my favorite for general annotation, and GoodReader my go to tool when there is something unusual I'm trying to do with a file, like zip it.