I have used Reader 5.1 for the past couple of years to fill in some forms. It will save the filled-in data when the form is closed despite the fact that's it's apparently not supposed to be able to (and save as, email, etc. all indicate that the data would be lost because I'm not allowed to edit the file). I always assumed it was in the pdf, but this year, I noticed something odd: I had to fill in two copies of the form with different data. After filling it in once, I copied the pdf and overwrote all the data in the fields with the new data. Now, whenever either one is opened, the first data displays briefly and is then overwritten with the second data.
The data is saved when it can't be.
This version of Reader doesn't even support saving the data, but it's saved anyway.
Data that was typed into one shows up in both.
Reader is not actually installed, but only extracted from the archive (works fine this way and doesn't muck my system up as much) so I can remove it when done (I use Foxit for all other pdfs).
Finally, the saved data will eventually disappear but the length of time appears to be volatile.
The form in question is an Alabama tax form if that helps (download link can be provided if you want it).
I want to separate the two lists of data and actually save it somewhere but I have no idea where it's saved. I believe the pdf got bigger after I added data but the modification date is unchanged, nothing obvious shows up when it's loaded into notepad and that doesn't explain how the second data showed up in the first file.