BASALT · JOURNAL
Compressing PDFs, predicted sizes and measured ones
Compression in a PDF means re-encoding images, subsetting fonts and removing redundancy. The image work dominates on anything scanned, and it is where the trade offs live.
The settings that matter
Downsampling resolution, which is quadratic and therefore the most powerful lever. Colour conversion, where colour to greyscale removes two thirds of the data. And JPEG quality, which is the setting most likely to damage a document because sharp text edges are JPEG's worst case.
Acrobat exposes these through preset profiles and a custom mode, which is the right shape for the problem.
Why predicted sizes are unreliable
The output depends on the content of the images rather than their dimensions. A page of dense text compresses differently from a photograph at identical settings, and a mixed document averages out to a number that describes neither.
That means an estimate offered before the work is a guess, and estimates that are presented confidently and then missed are worse than no estimate.
The guarantee worth asking for
Never return a file larger than the input.
That sounds trivial and is not. Re-encoding images that were already well compressed frequently produces larger output, particularly at high quality settings, and a compression feature that hands back a bigger file has failed at the one thing it was asked to do.
Measuring the real output before replacing anything allows the tool to fall back to the original when compression would not help. Basalt measures rather than predicts and will not return a larger file.
What compression does not do
It does not remove metadata, attachments, earlier revisions or hidden layers. A smaller file is not a safer file, and confusing the two is how documents get sent because they now fit rather than because they are clean.
For evidence
Compress a copy. Repeated JPEG generations accumulate artefacts around text, and a production that has been compressed twice is measurably worse than one compressed once. If size is the problem and the recipient can accept several files, splitting is lossless where compression is not.
Frequently asked questions
Why is PDF compression output size hard to predict?
Because it depends on image content rather than dimensions. Dense text and photographs compress very differently at identical settings, so a mixed document's result cannot be estimated reliably from its input size.
Can compressing a PDF make it bigger?
Yes. Re-encoding images that were already well compressed can produce larger output, especially at high quality settings. A tool that measures the real result before replacing the file can fall back to the original instead of returning something larger.
Which compression setting should I change first?
Colour conversion for scans of monochrome documents, which removes two thirds of the data with no loss of real information, then resolution. Leave aggressive JPEG quality reduction last, because it damages text edges before it damages photographs.
Does compression remove hidden data from a PDF?
No. Compression re-encodes images and leaves metadata, attachments, earlier revisions and hidden layers untouched. A smaller file is not a cleaner one.
Doing it in Basalt
Basalt is a native macOS PDF toolkit with eighteen tools in one window: merge, split, compress, OCR, Bates numbering, privilege logs, forms, signing, watermarks, comparison and more. Redaction destroys content rather than covering it, and an independent verifier re-opens every written file and proves the material is gone before the file is saved. If it cannot prove it, nothing is written. A one time $29 licence covers up to three Macs, it is free for the first 24 hours, there is no account, and the engine holds no network entitlement at all, which macOS enforces at the code-signature level. Download it at basaltformac.com.
Redaction that proves itself
Basalt destroys the content you mark, then re-opens the file it wrote and proves the content is gone before it saves anything. Your documents never leave your Mac.