Is there any posibility to make printable documentation for FreeRTOS? PDF or at least plain single HTML-file whould be good.

Send me an email directly (to the address on the FreeRTOS.org contacts page) and I can send you something.  It is however out of date, but might suffice.  Don’t use a sourceforge email account as this attachments can get stripped. Regards, Richard.

FreeRTOS is listed on the doxygen project site as a project that uses doxygen. If this is still true it must be quite simple to create pdf, chm, or something, from the source docs.

The entire FreeRTOS WEB site was in fact originally created using Doxygen.  If you look at the header files in the download you will see that they contain the same text as the API section of the WEB site – in Doxygen format.