BASALT · JOURNAL
Why your PDF looks blurry after compressing it
Compression on a scanned document re-encodes its images, and text is the worst case for the way JPEG works.
Why text suffers first
JPEG discards high frequency detail, which is exactly what a sharp black edge against white paper consists of. Photographs tolerate this because their detail is gradual. Characters do not.
The result is halos and speckle around letters, appearing long before a photograph in the same document would look damaged.
Whether it can be undone
No. Compression is lossy and the discarded detail is gone. If you compressed over your only copy, the original quality is not recoverable.
This is the reason to always compress a copy, and it is worth making a habit rather than a decision.
Getting size down without the damage
Greyscale before quality. If the pages are black text scanned in colour, converting to greyscale removes about two thirds of the data and discards nothing real. Try this first; it frequently makes further compression unnecessary.
Resolution before quality. Reducing 600 DPI to 300 removes about three quarters of the pixels and keeps text crisp. Below 300 both legibility and OCR accuracy fall away.
Quality last, and gently. This is the setting that causes the damage you are asking about.
Or split instead. Splitting is lossless. If the constraint is an attachment limit and the recipient can take two files, nothing needs to be degraded at all.
Repeated compression
Each pass compounds. A document compressed twice carries the artefacts of both, and productions that have been through several hands can be noticeably worse than the original scan.
If you receive a document that already looks poor, do not compress it further. Split it.
Frequently asked questions
Why does my PDF look blurry after compression?
Because JPEG compression discards the high frequency detail that sharp text edges consist of, producing halos and speckle around characters. Photographs tolerate the same setting far better.
Can I undo PDF compression?
No. Compression is lossy and the discarded detail is gone. This is why you should always compress a copy rather than the original.
How do I shrink a PDF without making it blurry?
Convert colour scans of monochrome documents to greyscale first, then reduce resolution to no lower than 300 DPI, and only then touch quality. Splitting the document is lossless and avoids the trade entirely.
Does compressing a PDF twice make it worse?
Yes. Artefacts compound with each pass, so a document that has been compressed several times is noticeably worse than the original. Do not re-compress a file that already looks poor.
Doing it in Basalt
Basalt is a native macOS PDF toolkit: eighteen tools in one window covering merge, split, page organisation, compression, OCR, passwords, Bates numbering, forms, signing, watermarks and comparison. Every file is processed on your Mac, and the engine that opens documents holds no network entitlement at all, which macOS enforces at the code-signature level. Redaction destroys content rather than covering it, and an independent verifier proves the material is gone before a file is written. A one time $29 licence covers up to three Macs, free for the first 24 hours. basaltformac.com, or brew install --cask chipmunk1101/tap/basalt.
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.