PDFora
Free forever · nothing uploaded

How Does PDF Compression Work?

PDF compression almost always means shrinking the images embedded inside the file, since that is what makes most large PDFs large. This tool finds each JPEG image, resizes and re-encodes it at a lower quality, then saves it back into the PDF, leaving text and vector graphics completely untouched.

  • No signup
  • No watermark
  • No file limit
  • Free at download

Step by step

What happenswhen you click Compress.

  1. The tool opens your PDF and finds every image stored inside it, in your browser, nothing uploaded.
  2. Each JPEG image is decoded, resized to a smaller maximum dimension, and re-encoded at a lower quality.
  3. The smaller image replaces the original inside the PDF. Text and vector graphics are never touched.

The short version

Images are the weight.Text almost never is.

A page of plain text takes up only a few kilobytes, even across dozens of pages, because text is stored as text, not as a picture of text. A single embedded photo at full camera resolution, or a scanned page saved as an image, can be several megabytes on its own. That is why two PDFs with the same page count can differ in size by a factor of a hundred or more, and why compression targets images specifically rather than the whole page.

What this tool does not touch

Text and vector graphics are left completely untouched at every compression level, so nothing about the words on the page changes, no blur, no loss of selectability. Pages, page order and content are untouched too, compression only changes how the existing images are encoded, never what is on the page.

Flatten vs compress

These solve different problems, so this site keeps them as separate tools. Flattening, on the PDF Editor, merges form fields, annotations or a signature into the page so they stop being separate editable objects. Compressing, here, recompresses the images inside the PDF to reduce file size. A form you filled in needs flattening before it prints correctly; a PDF that is too big to email needs compressing.

The question everybody asks

Is it really free?

Yes. No account, no trial, no watermark on the output, no cap on file size or page count, and no payment step between you and the download button.

Why this is free → How files are handled

FAQ

Questions peopleactually ask.

How does PDF compression actually work?

The tool locates the JPEG-encoded images inside your PDF, decodes each one, resizes it to a smaller maximum dimension, and re-encodes it at a lower quality setting. The smaller image data replaces the original inside the file. Text and vector graphics are left exactly as they were.

What's the difference between flatten and compress a PDF?

Flattening merges form fields, annotations or a signature into the page itself, so they can no longer be edited as separate objects, it is an editing operation. Compressing recompresses the images inside the PDF to reduce file size, it is a size operation. This site handles flattening on the PDF Editor and compressing here, they are separate tools because they do different jobs.

Does flattening a PDF reduce its file size?

Sometimes a little, if it removes redundant form or annotation data, but it is not built for that and the effect is usually small. If your goal is a smaller file, compressing the images is what actually moves the number.

Why does compressing a scanned PDF work so well?

A scanned page is essentially a photograph of a sheet of paper, often saved at a high resolution the eye does not need for reading text. Resizing and re-encoding that image is where most of the size reduction on a scanned document comes from.

Why does compressing a text-only PDF barely change the size?

Text stored as actual text, not as an image, takes up very little space to begin with, often a few kilobytes for many pages. There is little for an image compressor to work on, so the file barely shrinks, which is expected rather than a sign the tool is not working.

Does this tool touch every image format inside a PDF?

It recompresses images stored as JPEG, which is the common case for photos and scans. A few less common internal image formats are left exactly as they are, rather than risk corrupting them.

Related

Now that you know how,go shrink one.

Have a specific problem instead? Why is my PDF file so large? covers the usual causes. Dashed cards below are in build, they are not links yet.