By running ocrdjvu on the djvu files I've managed to make searchable versions of some of the ICL manuals.
To run OCR on a .djvu file the following complex command can be used:
ocrodjvu --save-script xxx.djvused xxx.djvu(Producing a djvused script that can be used to include the OCR text in the .djvu file. Alternatively used the --in-place option to modify the .djvu file directlty.)
Further information can be extracted from the OCRed text, notably the document outline (making it easy to jump to specific pages) and a clickable contents list.
The Debian versions of ocropus and tesseract are slightly out-of-date and they work very badly on 64bit systems. Debian bug #590672. The results on 32bit systems are acceptable.
The old version of ocropus has some problems recognising text that touches the edge of the page. Debian bug #575484. The Debian bugreport contains a couple of minor patches for ocropus/ocrodjvu that work around this bug.
ocropus seems to have problems recognising the large bold words which many manual pages start:
Branch on Double Indexing
This makes automatic generation of bookmarks something of a pig.
The gnome document viewing program,
evince knows how to select
text from an OCRed djvu file, but doesn't show the selection area
on the screen: Gnome bug
Bug 448739 - Evince cannot select text in djvu documents.
This appears to be fixed in more recent versions of evince.
Evince doesn't show the document outline in the "Index" sidebar: Bug 592806 - empty index for .djvu file .
$ djvused -f xxx.djvused -s xxx.djvu