Quick answer: a PDF is usually large because of the images inside it. You can make it smaller by lowering the image resolution and quality, and the best place to do that is where the PDF is created. Tools that shrink an existing file work by redrawing its pages, which brings trade-offs worth knowing before you press the button.

Why PDFs get big

  • Scans and photos. A scanned page is a full-size picture. A 50-page scan can be tens of megabytes.
  • High-resolution images. A slide deck with print-quality photos stores every pixel, even if it is only viewed on screen.
  • Embedded fonts. Several fonts add some size, but far less than images do.

A PDF that is mostly text, such as one saved from a word processor, is usually small already.

Ways to make a PDF smaller

  1. Re-export from the source. In the app that made the file, save or export again with lower image quality. This keeps text selectable and is the cleanest method.
  2. Shrink the images first. If you are building a PDF from photos, run them through the Image Compressor before you create the PDF.
  3. Send fewer pages. Split the PDF and send only what is needed.
  4. Redraw the pages as compressed images. This is what the Compress PDF tool does.

What redrawing pages does

The tool draws each page, saves the picture as a JPEG at a chosen quality, and builds a new PDF from the pictures. For a scan this can cut the size a lot, because the file now holds one moderate-resolution image per page.

The cost is that everything on the page becomes part of one flat image. Text can no longer be selected, searched or copied. Links and form fields are removed, and a digital signature stops being valid. Keep the original for any of those needs.

When compression makes a file bigger

A text PDF stores letters as compact instructions. Turning a page into a picture of text can take more space, not less. If the result is not smaller, keep the original. Toolbantu shows both sizes and warns you when the output is larger.

Choosing a level

  • Light keeps the most detail (about 144 DPI). Choose it if small print matters.
  • Balanced (about 108 DPI) suits reading on screen.
  • Strong (about 72 DPI) gives the smallest file, with softer fine print.