:fileviewer isn't for opening files, so you can drop that line.
It might be that you have another :filetype definition earlier in your vifmrc file. Does SumartraPDF appear on the first line of the :file menu? If not, reorder :filetype commands to have SumatraPDF above the rest.
Also, you probably want to use %"f instead of %f, because on Windows the first one works better with spaces and special characters.
Update (2020-06-14): backslashes are the reason, replace them with forward slashes:
filextype *.pdf "C:/Program Files/SumatraPDF/SumatraPDF.exe" %"f
This looks like a bug, need to check why it doesn't normalize slashes in this case. So thanks for the report.
Update (2020-07-04): fixed the bug.