I regret to inform you....

Adobe Reader 6 Message
The Problem
You just spent 20 minutes tracking down the right form, and another 30 minutes carefully filling it out in Adobe Reader.  You click the “diskette” icon to save your work, and a message pops up informing you that you can print your completed form, but cannot save the data typed into it.  Ugg.   How can that be you wonder?!  You just filled out the Form W-9 and saved it without any problem!

In case you hadn’t noticed – not all forms are created equal.
This blog discusses working with PDF forms when using Adobe Reader.  If you own any version of Adobe Acrobat, you can skip this post.

Flavorful Forms:
In general, forms come in 3 flavors.  Flat, Standard, and Rights Enabled.
A “flat” form is typically a scan of a paper form – and has not been enhanced with fillable fields.  If you're using Adobe Reader then your best option is to print-to-paper, fill out, and rescan. 
The second flavor is a form which contains form ‘fields’ which allow you to type information into the field, and easily advance to the next field with the tab key.  We’ll call this a standard fillable form and information can only be saved if you’re working with Adobe Acrobat.   If you only have Adobe Reader at your disposal, you may have a problem!

The third flavor of form is exactly like the second flavor, except that it has been enhanced to allow you to save your information when working with Adobe Acrobat or Adobe Reader.  This is called rights enabling  - and the only application available to enhance a form in this manner is Adobe Acrobat .  Note that there are no visual cues to distinguish a rights-enabled form except a minor wording change presented in Reader X...
You Cannot Save

You Can Save

The Solution
The IRS website is full of examples of rights-enabled forms which are forms that have certain options enabled – like saving your information when using Adobe ReaderIf all forms were fillable and rights-enabled, we would all be happily locating, filling in, and saving forms with Adobe Reader. 
However, even though Acrobat 6 saw the first introduction of enabling usage rights for PDF Viewers some years ago, standard forms still about so operate with caution.  #1 - do a "test" 'save before you spend too much time filling in a form.  
#2 - check out the Docs Corp formFiller solution, which works with Adobe Reader to make it smart enough to allow saving Standard fillable forms as well as Rights-Enabled fillable forms.  At $49 a copy Its less than a copy of Acrobat and faster than retyping the form as a Word document.
#3 - if you're working with a flat form,  you might consider recreating the form in MS-Word, or  investing in a copy of a PDF editing tool so that you can create a standard form which you could then fill and save with the pdfDocs formFiller application.   Or buy a copy of Adobe Acrobat so that you can add fields, and then save the form with rights enabled for your other users working with Adobe Reader.