After unregistering some problem packages (see above)
which contained '[pdf|ps].gz' file entries and running 'dhelp_parse -r'
to create a new pending list (/var/lib/dhelp/pending.list), I was
able to create a valid 'documents.index' file by manually running
index++ against it, and searching works fine.
As for solving the root cause, it seems the dhelp filters
('/usr/share/dhelp/config/swish++.conf') may not be handling 'compound
files' (i.e. compressed .pdf and .ps) correctly. This _should_ be
possible, per 'man swish++.conf':
"A file can be filtered more than once prior to indexing or extraction,
i.e., filters can be ``chained'' together. For example, if the
uncompression and PDF examples shown above are used together,
compressed PDF files will also be indexed or extracted, i.e., filenames
ending with one of .pdf.bz2, .pdf.gz, or .pdf.Z double extensions."
Studying and testing changes to the dhelp swish++.conf...
