PDF file size is primarily driven by the images embedded within the document. A scanned document, for example, is essentially a series of high-resolution photographs stored inside a PDF container. A single A4 page scanned at 300 DPI as an uncompressed image can be 25 MB or more. Multiply that across a 20-page document and you have a 500 MB file that no email server will accept.
Even text-based PDFs can become bloated. Embedded fonts, colour profiles, revision history data, form fields, and metadata all add to the file size. PDF files created by design software like Adobe InDesign often contain full print-resolution images even when the document will only ever be viewed on a screen.
PDF compression uses two main approaches. Lossless compression reorganises data more efficiently without discarding any information — the result is identical to the original but packed more tightly. Lossy compression reduces image resolution and quality, which produces smaller files at the cost of some visual fidelity.
For most everyday uses — emailing documents, uploading to web portals, sharing reports — lossy compression at 80–85% quality is imperceptible to the human eye and can reduce file size by 50–80%. For documents that will be printed professionally or require pixel-perfect image quality, use lossless compression or a higher quality setting.
Compress your PDF before emailing it if the file exceeds 10 MB (many email servers reject larger attachments). Compress before uploading to any government portal, application system, or document management platform that imposes a file size limit. Compress before adding to a website — large PDFs slow page load times and consume server bandwidth.
Do not compress a PDF if it contains legal signatures that might be invalidated by modification, if it is a master print file that will be sent to a commercial printer, or if it has already been compressed and further compression would degrade quality significantly.