Look at the newest version of mudraw. It is a command line tool that is from the MuPDF family of tools.

mudraw -o out.html -F html in.pdf

Use the newest version, if possible. It has gained quite a few new and additional features (it can do more than just PDF->HTML conversion):

$ mudraw
   Usage: mudraw [options] file [pages]
   -p -  password

   -o -  output file name (%d for page number)
   -F -  output format (default inferred from output file name)
         raster: png, tga, pnm, pam, pbm, pwg, pcl
         vector: svg, pdf, trace
         text: txt, html, stext

   -s -  show extra information:
         m - show memory use
         t - show timings
         f - show page features
         5 - show md5 checksum of rendered image

   -R -  rotate clockwise (default: 0 degrees)
   -r -  resolution in dpi (default: 72)
   -w -  width (in pixels) (maximum width if -r is specified)
   -h -  height (in pixels) (maximum height if -r is specified)
   -f -  fit width and/or height exactly; ignore original aspect ratio
   -B -  maximum bandheight (pgm, ppm, pam, png output only)

   -W -  page width for EPUB layout
   -H -  page height for EPUB layout
   -S -  font size for EPUB layout

   -c -  colorspace (mono, gray, grayalpha, rgb, rgba, cmyk, cmykalpha)
   -G -  apply gamma correction
   -I    invert colors

   -A -  number of bits of antialiasing (0 to 8)
   -D    disable use of display list
   -i    ignore errors

   pages comma separated list of page numbers and ranges

Update (April 2016)

The calling convention of the tool has been changed. It is still part of the MuPDF family, but you run it like this now:

mutool draw

If you're on Linux, try pdftohtml:

sudo apt-get install poppler-utils
pdftohtml -enc UTF-8 -noframes infile.pdf outfile.html

On MacOS (with homebrew) pdftohtml can be installed with:

brew install pdftohtml

The open source ebook converter Calibre can also convert PDF files to HTML and is available on MacOS, Windows and Linux.

You're unlikely to find a single offering that does all this, especially in the open source world. It's more likely that you'll end up relying on a mishmash of things, and may even need to chain some converters in order to get to HTML. (Eg PDF -> ps -> HTML)

OpenOffice supports conversion to HTML, and can be called from the command line.

http://pdftohtml.sourceforge.net/ looks reasonably good at converting pdf to html.

For Doc that is Word ML or OpenXML format it's conceivable that you could use XSLT transforms since both input and output formats are XML. I've seen some stylesheets floating around the net that do this, but YMMV.

Incidentally, why is there a specific requirement for open source? MS Powerpoint already supports save-as-HTML for example.