Compress PDF

What does "compress" actually mean here?

There are three settings. Lossless, the default, re-saves your PDF: it drops data nothing in the document refers to any more (common in files that were edited many times) and packs the file's internal objects into compressed streams. Nothing you can see changes. Balanced and Smallest also recompress the JPEG images inside the file - photos and most scans - at up to 150 or 100 DPI and a lower JPEG quality. That is lossy: those images lose some detail for good. Text, vector graphics and images that are not JPEGs are never changed.

Which setting should I choose?

Lossless if the file is mostly text. For a scan or a PDF made from photos, Balanced usually shrinks it a lot while staying sharp enough to print. Smallest is for reading on screen, for example to get under an email or upload limit. You can try another setting on the same file without adding it again.

Why were some images left as they were?

Only JPEG images are recompressed. Images stored losslessly - screenshots, diagrams, logos - would pick up blur and artefacts as JPEGs, and black-and-white scans are usually stored in a fax-style format that is already very compact, so both are left alone. So are CMYK and other print colour spaces, transparency masks, and any image whose recompressed version would not be smaller. The result lists how many were left and why.

Why did the tool say "no savings found"?

The file is already efficiently stored, or its size is in something the chosen setting does not touch - text, fonts, or images that are not JPEGs. PDFs exported from modern tools often have little to gain from a lossless re-save; if the file contains photos or scans, try Balanced.

Can I compress a scanned PDF?

Yes, when the scan is stored as JPEG images, which most colour and greyscale scans are: Balanced or Smallest will downsample and recompress them. Black-and-white scans are usually stored in a fax-style format that is already compact, and those are left unchanged.

Is the original file changed?

No. HerePDF only ever writes a separate output file - it never overwrites or edits the file on your device. Balanced and Smallest discard image detail permanently in the new file, so keep your original if you might need full quality later.