How to reduce PDF file size: what works, and what it costs you

Why PDF files are large

Open a PDF and you see pages, but the file is a container holding several kinds of data. Text is cheap: a page of plain text is usually a few kilobytes. Images are expensive. A scanned document or a brochure with photos is mostly image data, and that is almost always where the megabytes live. Embedded fonts come next; a font family with several weights, fully embedded, can add hundreds of kilobytes. Vector artwork from design tools, transparency effects, form fields, attachments and unused objects left by previous edits add more.

Image size follows from pixel count. As an illustration, an A4 page at 8.27 by 11.69 inches covers about 595 by 842 pixels at 72 dpi, 1,241 by 1,754 at 150 dpi, and 2,481 by 3,507 at 300 dpi. Uncompressed RGB at 3 bytes per pixel, that is roughly 1.5 MB, 6.5 MB and 26 MB per page. JPEG compression typically cuts that dramatically, but the pixel count still drives the result. Doubling the resolution quadruples the pixels.

Find out whether the weight is images, fonts or structure before choosing a method.

Your options, from least to most destructive

Pick the gentlest method that gets the file under your limit.

1. Export again at a lower image quality. If you still have the Word, Google Docs, PowerPoint or design file, the export dialog usually offers "minimum size" or an image-quality setting. This is the best option, because it recompresses from the original images instead of degrading an already compressed file.
2. Reduce the images before putting them in the document. Resize a 4,000-pixel photo to about the size it displays and convert large PNG photos to JPEG. The Image Compressor and Image Resizer tools do this in the browser.
3. Remove pages you do not need. The Delete PDF Pages tool drops them; this cuts size in proportion to what you remove.
4. Split the file into parts. If an email limit is the only problem, the PDF Split tool lets you send two attachments, though a recipient may prefer one file.
5. Optimise the structure. This tidies the file without touching images, and gains are modest on files that are mostly photos but can help on files with redundant objects.
6. Rebuild the pages as compressed images. This gives the largest savings on scanned or image-heavy files, at the cost of selectable text and quality loss.

What the Compress PDF tool does and does not do

The Compress PDF tool on this site runs in your browser and offers two modes. In "Rebuild as images" mode, each page is rendered to a picture at the resolution you set and saved as a JPEG at the quality you set; the pages are then assembled into a new PDF. Lower numbers give a smaller file. In "Optimise structure only" mode, the tool re-saves the document without turning pages into pictures, so text stays selectable and sharp, but the savings are smaller and can be zero if the file is already tight.

It is important to be clear about the limits. Rebuilding as images means the text is no longer selectable or searchable, links and form fields stop working, and the result is a raster, so text may look slightly soft when zoomed in. It will not do clever per-image recompression that keeps text as text; for that, a desktop tool with an "optimise" feature is the right choice. It also will not make a file bigger on purpose, but it can: a file that is already small and mostly text may grow when rebuilt as images. The tool shows the size before and after, so download only if the new file is smaller and still looks acceptable.

Compare before and after sizes, and keep the original; rebuilding as images is not reversible.

Choosing resolution and quality

If you use the image mode, resolution and quality are the two dials. A rough guide to the trade-off:

JPEG quality around 60 to 75 percent is usually where most of the saving happens before artefacts become noticeable on text. Try one setting, zoom in on the smallest text, and adjust. Do not set both dials to minimum at once; change one, check, then the other.

Fonts, transparency and other hidden weight

If your PDF has little imagery and is still large, look elsewhere. Embedded fonts are one cause: if a file embeds the entire font rather than only the characters used, the file carries thousands of unused glyphs. Many export tools have a "subset fonts" option that keeps only the used characters. Another cause is flattened transparency and high-detail vector art, where a single complex logo can have tens of thousands of path points. A third is leftover data: PDFs edited many times can keep old objects, and files can carry attachments, thumbnails, and form data. Saving a fresh copy through a clean export, or the structure-only mode, can discard some of that.

Scanned documents deserve special mention. A scan is a picture of a page, often saved at 300 dpi or more in colour. Re-scanning in black and white or greyscale at 150 to 200 dpi shrinks the file far more than any later compression will, and text still looks fine for reading.

A practical workflow

Start with what you control. If you have the source, export at lower image quality. If not, check what is in the file: open it, note whether it is scanned pages, a photo-heavy brochure or mostly text. For scanned or photo-heavy files, try image mode at 150 dpi and about 70 percent quality, then compare. For text-heavy files, try structure-only mode first, because keeping real text is worth more than a few extra kilobytes. If the file is still too big for an email limit, split it into parts or share a link through whatever storage you use. Do not repeat compression several times on the same file; each pass recompresses an already lossy JPEG and quality drops faster than size.

Frequently asked questions

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