How to convert pdf file to an odt file?

You could take a look at PDF Utilities (poppler-utils via Synaptic or apt-get) which includes pdftotext:

Poppler is a PDF rendering library based on Xpdf PDF viewer.

This package contains command line utilities (based on Poppler) for getting information of PDF documents, convert them to other formats, or manipulate them:
* pdfdetach -- lists or extracts embedded files (attachments)
* pdffonts -- font analyzer
* pdfimages -- image extractor
* pdfinfo -- document information
* pdfseparate -- page extraction tool
* pdftocairo -- PDF to PNG/JPEG/PDF/PS/EPS/SVG converter using Cairo
* pdftohtml -- PDF to HTML converter
* pdftoppm -- PDF to PPM/PNG/JPEG image converter
* pdftops -- PDF to PostScript (PS) converter
* pdftotext -- text extraction
* pdfunite -- document merging tool

Of course, success will depend on how the pdf file was generated. If you get what you want as a text file, you could then save that as an .odt file.

Edit: I forgot to provide the source for the quote. It's from the description tab in Synaptic for PDF Utilities (based on Poppler).

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.