How to create page margins in a ggsave() pdf file

I am trying to print a series of ggplot graphs in a multipage PDF. Everything works fine except I can't figure out how to define the page margins. I want to produce 8.5x11 pages with 1" margins on all sides. The following code produces a PDF page that is only 6.5x9. When printed on a normal sheet of paper, the margins are fine because the PDF is centered, but when viewed in a PDF viewer like Acrobat, there are no margins.

library(ggplot2)
library(gridExtra)

x <- c(5,7,2,4,9)
y <- c(2,10,15,8,14)
df <- data.frame(x,y)
myfunct <- function(i){
  p<-ggplot(df, aes(x=x,y=y*i)) +
    geom_point()
  return(p)
}
myGrobs <- lapply(1:10, myfunct)
page <- marrangeGrob(myGrobs, nrow=3, ncol=1)
ggsave("test pdf.pdf", plot = page, device = "pdf", 
        width = 6.5, height = 9, units="in")

We can use grid::viewport() to define the region of the page that we want to draw the plots.

First, create the grobs.

library(ggplot2)
library(grid)
library(gridExtra)

x <- c(5,7,2,4,9)
y <- c(2,10,15,8,14)
df <- data.frame(x,y)

myfunct <- function(i){
  ggplot(df, aes(x=x, y=y*i)) + geom_point()
}
myGrobs <- lapply(1:10, myfunct)
page <- marrangeGrob(myGrobs, nrow=3, ncol=1, top = "")

Then we can draw the plots in the predefined viewport:

vp <- viewport(x = 0.5, y = 0.5, 
               width = unit(6.5, "inches"),
               height = unit(9, "inches"))
cairo_pdf("controlled_margins_1.pdf", width = 8.5, height = 11)
for(i in seq_along(page)) {
  grid.newpage()
  pushViewport(vp)
  grid.draw(page[[i]])
  popViewport()
}
dev.off()

or as OP mentioned in the comments:

pdf("controlled_margins_2.pdf", width = 6.5, height = 9, paper="letter") 
for(i in seq_along(page)) { 
  grid.newpage() 
  pushViewport(viewport()) 
  grid.draw(page[[i]]) 
  popViewport() }
dev.off()

As you can see we have the right page size and right margins (the 10th plot obviously has empty space at the bottom):