{pdf} is specifically about the PDF file format. If your question is about producing PDF output, choose an appropriate tag such as {pdftex} or {ps2pdf}. If your question concerns PDF reader software, also use {viewer} or a reader specific tag such as {acrobat}, {evince} or {okular}.
pdf is specifically about the PDF file format, with topics such as bookmarks, links and meta information.
If your question is about producing PDF output, choose an appropriate tag such as pdftex or ps2pdf.
If your question concerns PDF reader software, also use viewers or a reader specific tag such as acrobat, evince or okular.
