Estimate the size of a PDF.
PDF size is almost entirely images. A page of text is around ten to fifteen kilobytes of compressed content streams, so fifty text pages barely reach a megabyte, while a dozen pages carrying two photos each can add several. Embedded fonts add a fixed overhead of a few hundred kilobytes regardless of length — significant for a two-page document, irrelevant for a two-hundred-page one.
Component sizes
Text KB = total pages x 12; image KB = image pages x images per page x average image KB
Total size
Total KB = text KB + image KB + font overhead + 30 KB of document structure
Downsample images to the output resolution — 150 dpi for screen, 300 for print — and use JPEG for photographs. That addresses the component that dominates the file.
Yes for anything sent for printing or archiving, since without embedding the layout depends on fonts the recipient may not have. Subsetting embeds only the glyphs used and cuts the overhead substantially.