How to convert a PDF document to an older version

Acrobat 9 Pro can be used to re-distill PDF 1.7 into any "lower" version of PDF. Look at the Distiller settings... Up to you to decide if it's "not too expensive".

Depending on the exact feature subset of PDF-1.7 used in your files, even (Free) Ghostscript may be able to do a good quality transformation to PDF-1.2, PDF-1.3 or PDF-1.4 file format version.

This is the Ghostscript command line to create a PDF-1.4:

 gs                        \
  -sDEVICE=pdfwrite        \
  -dCompatibilityLevel=1.4 \
  -o output.pdf            \
     input.pdf 

Note:
The more recent versions of Ghostscript can abbreviate the parameter -sOutputFile=... to -o .... It also implicitly also sets -dBATCH -dNOPAUSE. So it is much shorter to type and much easier to avoid typing errors.

If you're using Ubuntu 11.04 (or probably any Gnome distro) and can view the PDF in the built-in Document Viewer, you can then Print to file to save it as a PDF-1.5. (Save A Copy, in contrast, just creates a byte-identical copy of the file.) I did this today to convert a PDF from 1.6 to 1.5, so now I can use it with the FPDF lib.