evince letter_one.pdf &
# ...don't forget the "&"...
Other Linux PDF viewers include KDE's kpdf and the stone-age, but still
functional, xpdf.
Make Some Simple Changes to the Document
Kile
Enter some text in the document window within Kile,
then compile again (Build, Compile, PDFLaTeX)
and update the preview (assuming evince: View, Reload).
Command-Line
Use gedit, or your favourite text-editor.
Recompile the source file using the pdflatex command and then
preview the new version by reloading in the viewer — in evince:
View, Reload; in xpdf, right-click and select reload.