How to search recursively for a string in linux

Use grep instead of sed:

find /var/www -type f -print0 | xargs -0 grep -i 'old string'

From the way you phrase the question, it seems like you're not yet too familiar with grep. Read more about its options in its man page type: man grep at your command line.

update to answer comment -- try adding the -l option to show just file names. The -i makes the search case insensitive. The easy to use both is with a single dash: grep -il

Use find for recursive searches:

find -name '*.doc' -exec catdoc {} + | grep "specificword"

This will also output the file name:

find -name '*.doc' | while read -r file; do
    catdoc "$file" | grep -H --label="$file" "specificword"
done

_{(Normally I would use find ... -print0 | while read -rd "" file, but there's maybe a .0001% chance that it would be necessary, so I stopped caring.)}