Two modes, and why
Light: keeps the PDF as it is
Light mode rebuilds the document from its pages. Objects nothing refers to are left behind, as are page thumbnails, XMP metadata and private application data, and the result is saved with compressed object streams. Text stays selectable, links and form fields keep working, and images are not touched. On PDFs exported by office software or edited several times this can save a lot. On a tidy PDF it saves little, and the tool tells you rather than handing back a "compressed" file that is no smaller.
Strong: pages become images
Strong mode renders each page to a picture with pdf.js and stores it as a JPEG at the resolution and quality you choose. For scanned documents, which are already pictures of pages, this is often a huge saving with no visible difference. For text documents it also shrinks them, but the text is no longer real text: it cannot be selected, searched or read by a screen reader, and links stop working. The tool says so on the result. Use strong mode when the upload limit matters more than those things, and keep your original.
Hitting a target size
Tick Aim for a target size and enter the limit, for example 5 MB. The tool renders each page once at 200 dpi, then measures how big it would be at 200, 150, 110 and 80 dpi and at four JPEG qualities. It picks the highest resolution, then the highest quality, whose total fits under your target, and builds the file at that setting. If nothing fits, it uses the smallest setting and tells you, at which point splitting by size is the better answer.
Choosing settings by hand
- 110 dpi is enough for reading on screen and the smallest option.
- 150 dpi is a good balance; printed text stays crisp at normal reading distance.
- 200 dpi prints well and keeps small print legible.
- JPEG quality 60–75 suits most documents. Below 50, text edges show artefacts.
Australian portal limits
As reported, Centrelink's online account and Express Plus app accept PDF, PNG and JPEG, up to 10 files, with the total for each upload under 10 MB; Child Support's online account takes files under 5 MB each; and ImmiAccount is commonly quoted at 5 MB per attachment, with encrypted PDFs refused. We could not open every official page, so check the portal before relying on these. A good habit is to aim 10% under the stated limit.
- Compress a PDF to under 5 MB: settings that usually get there.
- Compress a PDF without uploading it: why it matters for identity documents.
Why some PDFs will not shrink
- They are already compressed. A PDF of JPEG photos is mostly JPEG data. Strong mode at a lower resolution is the only lever.
- Embedded fonts dominate. Small text-only PDFs with several embedded fonts may be mostly font data, which light mode keeps.
- Vector drawings. Detailed maps and CAD exports can be large; strong mode turns them into images, which may be smaller or larger depending on the drawing.
Large documents take time in a browser, a few seconds per page in strong mode on a typical laptop and longer on a phone. The progress bar shows the page being processed and Cancel stops at once.
On a phone
Phone scans are the usual reason for oversized PDFs, and the compressor runs on phones too. Strong mode renders pages one at a time, so memory use stays modest, but it is slower than on a computer: allow a second or two per page. If a phone struggles with a long document, split it first and compress the parts.
What "smaller" costs
Every compression choice is a trade. Light mode never loses anything you can see. Strong mode at 150 dpi and quality 70 is visually identical to the original for scanned documents read on screen, slightly softer when printed, and loses selectable text. At 110 dpi small print starts to blur. Look at the result at 100% zoom, especially at stamps, signatures and fine print, before you submit it.
Keep the original
Always keep the uncompressed file. If a portal later asks for a clearer copy, you can produce one without rescanning.
Privacy: the file stays on your device
Online PDF services upload your document to their servers, process it there and keep it for a while. This tool does the work in your browser. The file is read into the page's memory, processed by open-source libraries (pdf-lib for editing and pdf.js for rendering) in a background worker, and the result is offered back as a download. Nothing is sent anywhere, there is no account, and closing the tab discards everything. You can check it: open your browser's developer tools, watch the Network tab while you run the tool, and you will see no upload. After the page has loaded once it also works offline.