Empty pdf's

We have a large number of pdfs, handled via Edocman. They are indexed and we see them in Joomla Smart Search if they contain the searched phrase.
However if we View the Search results, a small number of pdf's have 'empty pages' although the number of pages is correct, i.e. they do not show the pdf. Searching through these 'empty pages' shows vaque where results are located. The Download is OK.
I hope you have an explanation and can offer a solution.