BASALT · JOURNAL
Compressing a large scanned PDF without wrecking it
Compression on a scanned document means re-encoding images, and every choice is a trade against legibility. Some trades are free and some are expensive, and they are not obvious from the names of the settings.
The free saving
Converting a colour scan of a black and white document to greyscale removes two thirds of the data and discards nothing that was in the original document. If the pages are typed text on white paper and they were scanned in colour, this is the first thing to do and it usually dominates every other setting.
The careful saving
Reducing resolution. 300 DPI is the working floor for documents that may be read closely or OCRed, and going below it degrades recognition accuracy quickly. Above 300, reduction is often free in practice, since the extra pixels rarely carry information for typed text.
Resolution is quadratic, so going from 600 to 300 DPI removes about three quarters of the pixels.
The expensive saving
Aggressive JPEG quality. Sharp black on white edges are the worst case for JPEG, so text develops halos and speckle around characters long before photographs would look damaged. On a document that may be read in court, that is a real cost.
Worse, it compounds. A production that has been compressed twice carries the artefacts of both passes.
Measure, do not predict
Predicted output sizes are frequently wrong, because the result depends on the content of the images rather than on their dimensions. A tool that measures the real output before replacing anything can also guarantee something useful: never return a file larger than the input, which is otherwise a genuine and common outcome when re-encoding already well compressed images.
What compression does not do
It does not remove metadata, attachments, earlier revisions, or hidden layers. Those are structural, not visual, and they are the parts that leak information. A file that is half the size is not a file that is safe to send, and the two questions should not be confused.
The rule for evidence
Compress a copy. Keep the original. A compressed scan is a derived work, and if anyone later disputes what a page said, the answer should come from the file as received rather than from the version that was squeezed to fit an email.
Frequently asked questions
How do I compress a scanned PDF without losing readability?
Convert colour scans of monochrome documents to greyscale first, which is usually the largest saving and discards nothing. Then reduce resolution no lower than 300 DPI. Leave aggressive JPEG quality reduction as a last resort, since it damages text edges before it damages photographs.
Why does compressing a PDF sometimes make it larger?
Because re-encoding already compressed images can produce larger output than the original, particularly at high quality settings. A tool that measures the real result before replacing the file can refuse to return anything larger.
Does compressing a PDF remove hidden data?
No. Compression re-encodes images and leaves metadata, attachments, earlier revisions and hidden layers untouched. Those are the parts that leak, and they need a separate sanitisation step.
Is it safe to compress a document that will be produced?
Compress a copy and keep the original. Compression of scanned text is lossy and repeated passes accumulate artefacts, so the authoritative version should always be the file as received.
Doing it in Basalt
Basalt is a native macOS PDF toolkit with eighteen tools in one window. It opens large documents without loading them into memory, renders pages on demand, and copies files into its engine in fixed-size chunks, so peak memory follows the chunk size rather than the file size. Redaction destroys content rather than covering it, and an independent verifier re-opens every written file to prove the material is gone before the file is saved. A one time $29 licence covers up to three Macs, it is free for the first 24 hours, 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.