Compress PDF

Why PDF files get so large

A PDF is a container. Inside it sit page descriptions, fonts, images, metadata and sometimes several older revisions of the same content. A ten-page text document exported from a word processor is often under 200 KB, while a ten-page scan can be 20 MB or more, because each page is a full-size photograph of paper. Knowing what fills your file tells you which kind of compression will help.

The biggest offenders are almost always images. A scanner set to 600 dpi produces a page of about 5,000 by 7,000 pixels, far more detail than a screen or an office printer can show. Photos pasted into a slide deck and exported to PDF keep their original resolution too. Embedded fonts, especially complete font files instead of the few letters actually used, add a smaller but real amount. Forms, layers and edit history from design software add still more.

The two ways this tool compresses a PDF

The tool offers two methods because there are two honest ways to make a PDF smaller, and they suit different files. Choosing the right one matters more than any slider.

• Lossless optimize repacks the file more efficiently and leaves text, images and quality untouched. Typical savings are modest, from nothing to around a third, and a file that was already exported carefully may not shrink at all. Use it for documents you may need to edit, search or copy from, and for files with small images.
• Reduce quality draws every page as an image and builds a new PDF from those images. This can shrink scans and image-heavy documents dramatically, but the text becomes part of the picture: it can no longer be selected, searched or copied, and links and bookmarks are lost. Use it for scanned pages, photo albums and forms you only need to read or print.

Choosing between 150, 110 and 72 dpi

Resolution is measured in dots per inch. An A4 page is about 8.27 by 11.69 inches, so at 150 dpi it becomes roughly 1,240 by 1,754 pixels, at 110 dpi about 910 by 1,286 pixels, and at 72 dpi about 595 by 842 pixels. The lower the number, the smaller the file and the softer the text.

• High (150 dpi) is the safe choice for documents that will be read closely or printed on an office printer. Body text stays sharp.
• Medium (110 dpi) suits reading on a laptop or tablet. Small print can look slightly soft when you zoom in.
• Low (72 dpi) gives the smallest file and is fine for a quick preview or a form you must upload under a strict size limit. Fine print may become hard to read, so check the result.

Compressing to an exact target size

Many forms state a hard limit, such as "PDF under 200 KB" or "maximum 1 MB", and reject anything larger. The target-size mode is built for that. You pick a limit, the tool first tries a lossless repack, which keeps text selectable, and only if that is not enough does it re-render the pages as images at 150, 110, 96, 72, 60 and finally 48 dpi, stopping at the first quality that fits. It tells you which step it reached, so you know how much detail was traded away.

If even the lowest step cannot reach the limit, the document simply has too much content for that size: a 40-page colour scan will not fit into 100 KB. Reduce the number of pages with Split PDF, remove pages that are not required, or ask whether the limit applies per file so you can send the document in two parts.

Meeting a size limit for email or an online form

Email services commonly cap attachments at about 20 to 25 MB in total, and many application portals, government forms and learning platforms accept only a few megabytes per file. Work backwards from the limit. If your file is 18 MB and the limit is 5 MB, you need to remove about 70 percent of it, which usually means the image-based method at Medium or Low quality.

Compress once from the original rather than compressing an already compressed file. Each pass through a lossy method throws away more detail, so repeated compression looks worse than one careful pass. If a single scan-heavy file will not fit even at Low, use Split PDF to send it in two parts, or reduce the number of pages you send.

Make PDFs smaller before you create them

The cheapest compression happens at the source. When you scan, choose 150 to 200 dpi and grayscale for text documents; colour and 600 dpi rarely help readability but multiply the size. Before placing photos in a document, shrink them with the Image Compressor or Image Resizer, because a photo displayed at 10 centimetres wide does not need 6,000 pixels across.

When you export from Word, Pages or Google Docs, pick the "minimum size" or "standard" option instead of "high quality print" unless you need print-ready output. Design tools have similar presets. For a longer walk-through of the whole process, read how to compress a PDF and how to reduce PDF file size without losing quality.

Check the result before you send it

Open the compressed file and look at the pages that matter: the smallest text, any signatures or stamps, tables of numbers and photographs. Confirm the page count matches the original. If you used the image-based method, remember that search and copy will not work; keep the original for anything that needs them, and consider running OCR if you need text back.

Privacy is part of the design. The file is opened and rebuilt inside your browser tab, so contracts, IDs and medical documents are never uploaded to a server. Very large files are limited by the memory of your device rather than by a server quota, which is why there are size and page limits on the image-based method.