Why PDF files get so large

Most oversized PDFs are that way because of images. When you export a document from Word, InDesign, or a design tool, the images inside are often embedded at full resolution — sometimes 300 DPI or higher — even if the file is only ever going to be read on a screen. A single full-page photo at print quality can push a PDF past 5 MB by itself. Fonts, embedded ICC color profiles, and layers from the source application add to that.

Scanned documents are a different story. When you scan a paper document, each page becomes a raster image. A 50-page scanned report can easily hit 40–50 MB even though the actual content is just black text on white paper, because none of that content is stored as text — it's all pixels.

What the four presets actually do

• Screen — targets 72 DPI, the minimum resolution that looks acceptable on a screen. Images get heavily re-compressed. Use this if you just need to email something quickly and nobody's going to print it.
• Ebook — 150 DPI with moderate compression. A good middle ground: noticeably smaller than the original but still sharp enough to read on tablets and laptops. This is the preset most people want.
• Printer — 300 DPI, light compression. Suitable for home or office printing. The file will be much closer in size to the original because it keeps enough data to reproduce fine detail on paper.
• Prepress — 300 DPI with full color fidelity and embedded ICC profiles preserved. For professional print jobs where color accuracy matters. Don't use this just to compress — it might even increase file size if the original was already compressed aggressively.

When compression won't help much

If the PDF is mostly text with no images — think a 10-page contract — compression will shave off some overhead, but don't expect a dramatic result. The file is already small. Similarly, if the PDF was already compressed once before, compressing it again usually doesn't recover much. You're essentially re-compressing already-lossy data.