Compress PDF to 500 KB

When a PDF under 500 KB helps

• Multi-page scans such as signed contracts or forms
• Brochures, menus and price lists with photos
• PDFs you offer for download on a website

As soon as you drop a PDF, the tool shows how much of the file is images. If images make up most of it, 500 KB is mostly a question of resolution. If they make up little, the text and fonts decide, and those stay as they are.

For a PDF on a website, the limit is yours to choose. A smaller file is less to download for every visitor, so pick the smallest target at which the pages still read well.

Budget per page at 500 KB

The whole file has to fit into 500,000 bytes (4,000,000 bits), pages, images, fonts and structure together. In a scan, the page images take nearly all of that. A single Letter page at 150 dpi, the Medium level, is 2,103,750 pixels, which leaves 1.9 bits per pixel at 500 KB.

The last two columns show the highest resolution on the tool's list at which a full scanned Letter page still gets that many bits per pixel. Quality bands from section 4.5 of Gregory Wallace's paper on the JPEG standard, which rates color images of moderate detail; he notes that results vary with the content, so read them as a cautious guide for scans. A4 is about 3% larger than Letter.

The steps the tool tries for 500 KB

The tool does not guess from a table: it compresses the real file and measures it. These are its ten steps, from mild to strong, with the pixel size of a full Letter page at each resolution and the number of such page scans that 500 KB holds at 0.5 bits per pixel.

Which PDFs fit in 500 KB

A one-page scan keeps 0.5 bits per pixel, which Wallace's guide rates good to very good, up to 200 dpi. Ten scanned pages still get there at 85 dpi. A PDF of text, like a letter or an invoice exported from a word processor, has no page images, so it fits or not depending on its fonts and text, and this tool copies those unchanged.

Some images stay as they are, because recompressing them would change them or gain nothing: images under 12 KB (12,288 bytes), black-and-white scans stored with fax-style compression (CCITT or JBIG2), JPEG 2000 images, images with a stencil or color-key mask, CMYK images, and flat graphics like logos and charts unless they shrink a lot in pixels. A recompressed image also has to be at least 10% smaller than the original, or the original is kept.

When 500 KB is out of reach

• If the PDF is already at or under 500 KB, the tool says so and you can send the original as it is.
• If it has no images the tool can recompress, it tells you right away that 500 KB is out of reach.
• If even the strongest step, 50 dpi at JPEG quality 30, stays above 500 KB, you see the smallest size it reached. When text, fonts and other parts alone take more than 500 KB, the message says how much. If that smallest version is at least 3% smaller than the original, you can still download it.

Then send fewer pages or split the document: cut out pages with Split PDF or drop the ones you do not need with Delete PDF Pages, and compress each part.

500 KB in bytes

Here 500 KB means 500,000 bytes. A system that counts in 1,024-byte units shows the same limit as 488.28 KB (0.477 MB); a file at or under 500,000 bytes is under both.

Gmail allows 25 MB per message for personal accounts. Attachments travel Base64 encoded, which turns every 3 bytes into 4 characters, so a 500 KB PDF takes about 666,668 bytes in the message. That makes 37 or 39 such files per message, depending on whether 25 MB means 25,000,000 or 25 × 1,048,576 bytes. Counting the plain files instead gives 50 or 52.

Getting a PDF under 500 KB

Drop a PDF above. The target is already set to 500 KB. Press Compress PDF and the tool first runs its strongest step to check that 500 KB can be reached. If it can, it searches its steps for the mildest one that fits, from 300 dpi at quality 85 down, which takes three or four more runs. Text, fonts, links and form fields are copied unchanged. When the result uses 72 dpi or less, the tool asks you to zoom in and check that scanned text is still easy to read.

Need a different limit? One size down is 300 KB, one size up 1 MB. Type any size into the target field, or use the PDF compressor with Low, Medium or High compression. To combine files first, use Merge PDF; to turn photos of a document into a PDF, JPG to PDF. For a photo or a scan saved as an image, compress the image to 500 KB instead.

Sources

• Gmail Help: attachment size limits
• RFC 2045, section 6.8: Base64 content-transfer-encoding
• Gregory K. Wallace: The JPEG Still Picture Compression Standard, section 4.5 (copy hosted by the National University of Singapore)

Frequently asked questions