pdf import not indexed

[hartmut]

I have imported several pdf-files in one UR-Database.
The pdf- files were downloaded from different sites in the net.
Now I see in the Item Attributes window that some of the files are correctly indexed and others have only 3 keywords.
The files which are not indexed are all from the same site in the net.
Do anyone here have an idea about the reason and/or suggestion to resolve this problem.
As the files are in the same UR-Database and I did not change the option for the Import it must be something witeh the pdf-files
Thank you


Hartmut