I was annoyed by the lack of a free PDF to ODT converter too. I didn't even need anything complicated. Just a tool that generates ODT files that I can then annotate in LibreOffice (e.g. to fill out forms).
I know how to do this manually, by converting the PDF document into graphics files and then importing them into LibreOffice, but that gets tedious quite fast.
So, I finally wrote a quick little shell script that does all the required steps automatically. You can find it at https://github.com/gutschke/pdf2odt
It can take any number of PDF and image files as input and generates a ODT file that can be opened and edited in LibreOffice. Images show up as page background, so you can write over them freely. Each image is associated with its own page style. Keep that in mind, when inserting page breaks and adjust the page style as necessary.
I tested the script on both Linux and Mac. Given that it only needs a handful of reasonably standard tools, it should be quite portable.