At the beginning I had only vague keywords and got no useful searching results in return. I tried to launch LibreOffice Writer to create a simple test file and it was printed successfully, but after saved as PDF the printing was failed. Then I noticed that the documents which had failed in printing were also all PDF files. So the problem could be the file type.
I used PDF as one of the searching keywords and found some bug reports of Ubuntu. I followed some suggestions in one of the threads  and got little success.
The approach of updating cups-filters with precise-proposed didn't work to me . Actually I didn't see any update packages after I enabling the precise-proposed option.
The working one was changing settings of the printers via command line . I made one of the printer worked with PDF documents by using settings as the following:
$ lpadmin -p Hewlett-Packard-HP-LaserJet-P3005 -o pdftops-renderer-default=gswhere Hewlett-Packard-HP-LaserJet-P3005 is the printer name. I deleted the second setting by using
$ lpadmin -p Hewlett-Packard-HP-LaserJet-P3005 -o pdftops-max-image-resolution-default=0
$ lpadmin -p Hewlett-Packard-HP-LaserJet-P3005 -R pdftops-max-image-resolution-defaultand the HP printer still worked fine when printing PDF documents.
The other Xerox printer, however, still didn't work after changing the settings.
 Printing on PostScript printers (or printers with PostScript-based driver) not working
 #20 of the above thread
 #17 of the above thread