03 / HOW TO COMPRESS A
How to compress a PDF
Go to the Compress PDF page and upload a single PDF file. The file can be up to 50 MB and contain up to 500 pages. Choose a compression profile: Low, Balanced (the default), or Strong. The tool runs on the server after the upload completes. While it works, the page shows uploading, then queued, then running with a percentage. When the job finishes, you can download the result.
The tool does not rely on the file name. It reads the actual bytes to confirm the file is a PDF. If you rename a non-PDF file with a .pdf extension, it is rejected before any processing starts.
04 / WHAT EACH COMPRESS
What each compression profile does
Low is lossless: it only restructures the PDF. It uses object streams, maximum deflate, and a linearised layout for faster opening, but never touches images. Nothing visible changes, so you get the same quality in a smaller file, if the structure allows it.
Balanced is the default middle ground. It does everything Low does and additionally shrinks embedded images to at most 1800 pixels on the long edge at JPEG quality 78. This usually reduces size with little visible difference, but it is a lossy step.
Strong goes further: it re-encodes images at up to 1200 pixels and quality 58, and also re-encodes losslessly stored images. This produces the smallest result but can visibly reduce image quality. Use it when you need maximum size reduction and can accept quality loss.
Each profile is a ceiling, not a fixed recipe. The tool tries every setting up to that ceiling and keeps the smallest valid result. That means Strong can never return a bigger file than Balanced would for the same input.
05 / HOW THE RESULT IS
How the result is verified
Before any candidate file is accepted, it is re-opened and its page count is checked. The original file is also one of the candidates: if no compressed version is smaller and valid, the original is returned unchanged. The tool also scans the uploaded file for malware before anything opens it, so an infected file never reaches the compression engine.
The produced file is parsed before it is offered for download. If a job silently produces something broken, the tool fails instead of handing over a corrupt file.
06 / WHAT IF THE FILE I
What if the file is not smaller?
Some PDFs are already compressed efficiently. If no profile produces a smaller valid file, you get a warning: OUTPUT_NOT_SMALLER. In that case, the original file is returned to you unchanged. This is not an error—it means the file could not be reduced further.
If you see this warning, you could try a stronger profile, but the tool already tried the best it could up to that profile's ceiling. If Balanced or Strong still cannot beat the original, the file is likely already at its practical size limit.
07 / THINGS TO KNOW ABO
Things to know about compression quality
Low compression never changes the visible content. Balanced and Strong re-encode images, which reduces quality. The degree of quality loss depends on the original images. Text and vector graphics are not touched by any profile, so pages with mostly text will see no quality degradation even with Strong.
If you need to preserve exact image quality, use Low. If you need a much smaller file for emailing or uploading, Strong may be better even if images lose some sharpness. You can decide after seeing the resulting file.
FAQ
Common questions
Will compressing a PDF reduce its quality?
It depends on the profile you choose. Low is lossless and does not change the visual content. Balanced and Strong reduce image quality by re-encoding images at lower resolutions and JPEG qualities. Text and vector elements are not affected.
What file size can I compress?
You can compress a single PDF file up to 50 MB and up to 500 pages. Larger files or longer documents are not accepted, regardless of the file name.
What does the OUTPUT_NOT_SMALLER warning mean?
It means the tool could not make the PDF any smaller, so it returned the original file unchanged. This can happen if the PDF is already well compressed. You can try a stronger profile, but if it still does not help, the file is probably at its minimum size.
Will my PDF look the same after compression?
If you use the Low profile, yes, it will look identical. With Balanced or Strong, images may look slightly softer or have artifacts. Text and vector graphics remain crisp. The tool tries every setting up to the chosen profile and picks the smallest valid result, but it cannot avoid quality loss if the profile demands it.
Can I compress multiple PDFs at once?
No, this tool accepts exactly one PDF file per job. You must run a separate compression for each PDF.
Why was my file rejected even though it had a .pdf extension?
The tool decides the file type by reading the file's actual content, not by its name. If the file is not really a PDF, it will be rejected. Renaming a file to .pdf does not fool the tool.