Three things drive PDF size. First, embedded images – page scans, photos, logos. When their DPI exceeds 150–200, the file is far larger than needed for screen or print. Second, embedded fonts: PDF stores a font subset for each typeface, and many fonts mean many megabytes. Third, hidden layers and metadata – comments, revision history, page thumbnails.
PDF compression lowers image DPI to 96–150 and applies JPEG compression with adjustable quality. For scanned documents this typically yields 3–10× reduction with no noticeable loss in readability. Text-only PDFs are already compact – compression gains little.
Rule of thumb: for email, choose 'Medium' quality (72–96 DPI). For archiving or printing, use 'High' (150 DPI). If you need to fit a size limit (e.g. 5 MB for a portal upload), compress, check the result, and step down quality if needed.