A little more searching has shown that the problem is not in FPDF but is in the fields I'm sending to the FPDF routine. If the string in $paragraph[2] contains an apostrophe the string is truncated.
echo "<input type='hidden' name='paragraph2' value='$paragraph[2]'>";
I can, of course, always change the apostrophe to eg %%% before posting and then change %%% back to an apostrophe after posting but there must be a more correct solution.
Any help appreciated.