You always need to
listen to a song to get the strum pattern. PVG sheets won't tell you, because they don't regard it as important (because it isn't
). Occasionally you might see distinctive chord riffs notated (showing the rhythms), but not strum patterns throughout a song.
If there are no distinctive rhythms shown in the accompaniment - and you haven't heard the song before, and can't find a recording - just use a simple generic strum pattern to begin with. That means downstrokes on every beat, possibly with accents on 2 and 4 (if it's a normal 4/4 pop/rock song), possibly with occasional upstrokes (between the beats, obviously).
If the tempo is slow, downstrokes on every 8th may be possible and advisable, if it feels uncomfortably slow to play downstrokes only on the beats. Of course you still mark the beats (quarter notes) with accents; the inbetween downs on the 8ths may not be marked at all, they're only there to keep the rhythm solid.
Chord rhythms are rarely related to the rhythms of a vocal melody. It's normally best to just keep the beat.
IOW, you count "1-2-3-4" with your RH downstrokes; and if the tempo is slow enough you count "1-and-2-and-3-and-4-and" all with downstrokes.