Reduce the size of a PDF without uploading it

What compressing a PDF actually does

Most of the weight in a PDF comes from images: photos, scans and screenshots. The compressor finds the embedded JPEG images, re-encodes them at a lower resolution and quality, and writes the file out again with a more efficient internal structure. Text and vector graphics such as charts and logos are not touched, so they stay sharp at any zoom level and the text remains selectable.

When the work is done you see the original size, the new size and the percentage saved. If the result is not actually smaller, you get your original back together with a note that the file was already well optimized, instead of a bigger file presented as a success.

There is one optional, stronger mode. With the Maximum compression preset you can choose to convert pages to images. That can shrink text-heavy scans much further, but the text is no longer selectable or searchable, so the option is off by default and comes with a warning.

Image resolution, quality and file size

An image in a PDF is a grid of pixels, and the file grows with the number of pixels and with how much detail is kept for each of them. A phone photo of 4000 by 3000 pixels placed on an A4 page holds far more detail than a screen or an office printer can show, which is why lowering its resolution saves so much while looking almost identical.

Two settings control the result. Resolution decides how many pixels are kept; quality decides how aggressively each JPEG block is simplified. Resolution usually has the bigger effect on size, quality mostly affects how clean edges and gradients look. Pushing both to the limit produces the smallest file but visibly softer pictures.

• Reading on a screen: around 100 to 150 pixels per inch is comfortable, so strong reduction is rarely noticeable.
• Office printing: text and vectors stay sharp; photos may look slightly soft at aggressive settings.
• Photo prints or archival scans: prefer High quality, or keep the original untouched.

Choosing between the three presets

Start with Recommended. It aims for a solid reduction while keeping photos comfortable to look at, and it is the right answer for most email and upload situations. Only move to another preset if the result is too big or too soft.

• Recommended: balanced size and quality, the default for sharing and uploading.
• Maximum compression: for strict size limits. Images become noticeably softer, and the optional convert-pages-to-images mode can shrink text-heavy scans further at the cost of selectable text.
• High quality: a modest saving with images kept close to the original, suited to documents that will be printed or examined closely.

Why some PDFs barely shrink

If the tool says your file was already well optimized, that is usually accurate. A report exported from a word processor is mostly text and vector shapes, which take little space, and there is nothing to re-encode. Images already saved at a modest resolution, or by a previous compression pass, have little left to give. Images stored in formats other than JPEG are also not re-encoded, so they contribute little to the saving.

Fully embedded fonts and file attachments can also account for weight. When a file will not shrink enough, the options are to split it into parts with Split PDF, remove pages you do not need with Delete PDF Pages, or, for a scan where text selection does not matter, try the convert-pages-to-images option under Maximum compression.