I'm trying to reduce the file size of a multipage pdf of a magazine article. When I use the "Reduce file size" function (via Save As => Quartz Filter => reduce file size), the text is unreadable. So, as described in MacWorld's article "Preview's Hidden Powers," I want to use the "Adjust File Size" function described in the Help file pasted in below. But when I select all the pages of the pdf in the sidebar, as instructed, the "Adjust file size" command is greyed out. In fact, I can't find any way to make it NOT be greyed out. Is the problem that I'm trying to reduce a pdf document, not an "image"? What am I doing wrong?
Thanks in advance for any help.

Here's what it says in the Help file. The "Adjust Size" command is always greyed out.
"Reducing an image’s file size
You can reduce the file size of an image without reducing its dimensions, if you need to save disk space or download an image more quickly. The reduced image will contain less detail than the original.
To reduce an image’s file size without changing its dimensions:
Choose Tools > Adjust Size.
Select Resample Image.
Enter a smaller value in the Resolution field.
The new size is shown at the bottom of the dialog.
To resize multiple images at the same time, display the images in the same window, select them in that window’s sidebar, and then choose Tools > Image Size."