Scaling pages without reflowing them
Resizing a PDF is not like resizing a word processor document. There is no reflow: the text does not re-wrap to the new width, because the file records glyphs at fixed coordinates rather than a paragraph that could be laid out differently. Changing page size scales the existing content geometrically, exactly like enlarging or reducing on a photocopier.
That means proportions are preserved and nothing is lost, but relative sizes stay relative. Scaling A4 down to A5 halves the area and makes 11-point text behave like 7.8-point text — legible for a competent reader, tiring over a long document, and genuinely difficult for anyone with reduced vision.
It also means margins scale. A document with generous 25-millimetre margins reduced to A5 ends up with 17.7-millimetre margins, which is usually fine. Going the other way, an A5 document enlarged to A4 gets margins that look wastefully large, because they were designed for the smaller sheet.
Why A-series resizing is unusually clean
The ISO 216 A series has a property no other paper standard shares: every size has an aspect ratio of exactly 1:√2, and halving the long side produces the next size down with the same ratio. A4 folded in half is A5; two A4 sheets side by side make A3. Scaling between any two A sizes therefore needs no cropping and leaves no letterboxing.
The scale factor between adjacent sizes is √2, about 1.414, or 70.7 percent going down — which is why photocopiers have a 71 percent button. Two steps, A4 to A6, is a factor of 2 exactly.
US Letter has no such property. Letter is 8.5 × 11 inches, a ratio of about 1:1.294, so converting between Letter and A4 always involves either a small crop or a small margin. A4 is narrower and longer, which is why documents crossing the Atlantic tend to lose a strip at the side or gain one at the foot.
Size on disk is a different problem
Changing page dimensions barely changes file size, which surprises people expecting a smaller page to mean a smaller file. The content is described in vectors and embedded images whose data is unchanged — you have altered the coordinate space, not the payload. An A4 document scaled to A5 is very nearly the same number of bytes.
Reducing file size means acting on what actually occupies it, which for most documents is images: downsampling them to a resolution appropriate for the output, and re-encoding photographs as JPEG at a sensible quality. Fonts are the other contributor, and subsetting so only the glyphs used are embedded typically saves a great deal in documents using large font families.
The PDF compressor does that work. Use resize when you need different physical dimensions, and compress when you need fewer bytes — they are unrelated operations.