Where the weight in a PDF comes from
A PDF is a container, and its size depends almost entirely on what has been put inside. Plain text and vector drawings are tiny: a 50-page contract typed in a word processor often weighs 200 to 500 KB. Images are what make PDFs heavy.
The usual suspects:
- Scans. A page scanned at 300 dpi in colour is roughly 2500×3500 pixels. Saved with light compression, that can be 1 to 3 MB per page, so a 20-page scanned agreement easily passes 30 MB.
- Phone photos placed into documents. A dozen 12 MP photos at full resolution carry far more pixels than any screen or printer will use.
- Presentations exported to PDF. Background images and screenshots are often stored at their original size on every slide.
Compression pays off on these. If your file is mostly typed text, it is probably already as small as it will get.
Two different engines on one page
The page offers two ways to compress, and they work very differently.
In the browser
The default path keeps the file on your device. Every page is drawn to an image at about 108 dpi, saved as a JPEG at the quality you choose on the slider (30 to 95, default 70), and placed into a new PDF. This is very effective on scans, because a 300 dpi colour scan carries roughly eight times as many pixels as a 108 dpi render.
The trade-off: the whole page becomes a picture.
- Text can no longer be selected, searched or copied.
- Links, form fields and bookmarks do not survive.
- Fine print below about 7 pt can look soft when zoomed in.
If the rebuilt file comes out larger than the original, which is typical for typed documents, you get the original back instead of a worse result.
On the server
The server option runs Ghostscript, a long-established PDF engine. Instead of flattening pages, it rewrites the document and downsamples only the embedded images, so text stays real text, links keep working and the file remains searchable. The slider picks one of three standard profiles:
- Below 50: the
screenprofile, images reduced to around 72 dpi. Smallest files, fine for on-screen reading only. - 50 to 84: the
ebookprofile, around 150 dpi. A good balance for email and sharing. - 85 and above: the
printerprofile, around 300 dpi. Keeps print quality, saves less.
Without an account you get one server run per day for files up to 500 MB; Premium removes the daily limit and accepts files up to 4 GB. Uploaded files and results are deleted automatically about an hour after the job starts.
Picking a quality value
For browser compression, these ranges work in practice:
- 70 (default): scans and photo pages stay comfortable to read; a typical 30 MB scanned document often lands in the 3 to 6 MB range.
- 50 to 60: noticeably smaller, with slight JPEG blur around text edges. Fine for receipts, IDs and forms you only need to submit.
- Below 45: halos and blockiness show on photos and coloured backgrounds. Use it only to squeeze under a strict upload limit.
- 85 to 95: little visible change and smaller savings; useful when the PDF contains photos you care about.
For mixed documents with typed text, try the server first: it keeps the text sharp and usually gives a better size-to-quality ratio.
When a portal caps uploads at 2 MB or 5 MB, start at 60 and only go lower if the result is still too big.
When the file will not shrink much
- The PDF is mostly text. Rasterising text in the browser makes the file bigger, which is why the original comes back unchanged. The server engine may trim a little by recompressing streams and subsetting fonts, but do not expect large gains.
- It was already compressed. Files exported with a "smallest size" preset have little left to remove.
- It is password-protected. Encrypted files need to be unlocked first, using the password you already have.
Getting the smallest useful file
- Remove what you do not need before compressing. Dropping blank scan pages or appendices with delete PDF pages saves more than any quality setting.
- Compress, starting at the default value, and compare the before and after sizes shown next to each file.
- Check the page with the smallest print at 100% zoom before sending.
If browser compression left you with a document that is no longer searchable, running it through OCR PDF adds a text layer back on top of the page images.
Several PDFs of up to 100 MB each can be compressed in the browser in one go.