A PDF can become smaller while losing the fine image detail that made it useful. A scanned label, faint diagram or small rasterised text may be more revealing than the first page’s large heading. Define what must remain legible before adjusting a filter to meet a size target.

Preserve the original PDF and make a distinctly named candidate copy. Choose two or three demanding pages for the initial comparison: one with small image text, one with thin lines and one with tonal detail if those features occur in the document.

Keep the filter experiment identifiable

In ColorSync Utility, Apple documents creating filters in Filters or duplicating a locked filter before editing its copy. Image Effects components control image size, bit depth, interpolation and compression. Expand the relevant component and change only the setting needed for the trial; give the copy a recognisable name.

Domain Information controls whether a filter is available in apps, PDF workflows or the Print dialog. If the filter is absent from the intended route, inspect that scope instead of repeatedly changing image-quality settings that do not govern availability.

Apple’s PDF workflow opens the document through File, Open, then chooses a filter from the menu at the bottom. The window previews its effects without modifying the file. Use the page navigation arrows to inspect other pages; Apply saves the changes. Confirm you opened the candidate copy before applying.

Judge the difficult content before accepting the size

Compare the selected demanding pages with the untouched source at a useful reading scale. Check complete characters, line intersections and meaningful shaded regions. A blur that seems minor in a photograph can erase a digit or join two lines in a technical document.

After applying the candidate filter, reopen that saved file and check the same content again. Record its size separately from the readability judgement. Meeting an upload limit is useful only if the recipient can still interpret the document’s required information.

If the candidate fails, revise one filter setting and produce another clearly identified copy from the original. Repeatedly processing an already reduced file makes the source of degradation harder to establish. Keep the accepted filter settings with the delivery record so a revised PDF can be assessed consistently, and retain the original for any future requirement that needs its greater image detail.

References: Apple: custom Quartz filters; Apple: applying PDF filters.