BASALT · JOURNAL
When a PDF refuses to get smaller
Compression re-encodes images. If the images are already well compressed there is nothing left to take, and the result can be larger than the input.
Why it can grow
Re-encoding an already compressed JPEG at high quality produces a new JPEG that is often bigger, because the second encoder cannot know what the first discarded and encodes the artefacts as though they were detail.
A tool that measures the real output before replacing the file can fall back to the original instead of handing back something worse. Many do not.
Why nothing happens
If the document is mostly text, the images are not the problem and image compression has nothing to work on. A text heavy PDF is already small, and if yours is not, the size is coming from somewhere specific:
An embedded font set, which can be megabytes if the full font was embedded rather than a subset.
A logo or letterhead image repeated on every page. If it is embedded once and referenced it costs little; if it was pasted onto each page separately it is stored many times.
Attachments. A document can carry whole files inside it, and they do not appear on any page.
Finding out where the size is
qpdf --show-npages document.pdfDivide the file size by the page count. Over roughly 100 KB a page means images and compression should help. Well under that with a large file means something structural, not pixels.
What to do instead
For structural bloat, strip rather than compress: remove attachments, drop unused resources, and rewrite the file as a single generation to discard earlier revisions.
For scanned documents, greyscale conversion and resolution reduction do far more than quality reduction, and they cost less legibility.
For a hard limit that compression cannot reach, split. It is lossless where compression is not.
Frequently asked questions
Why does compressing my PDF make it bigger?
Because re-encoding already compressed images can produce larger output, especially at high quality settings. A tool that measures the real result can fall back to the original rather than returning something larger.
Why does compression do nothing to my PDF?
Because the document is mostly text and there are few images to re-encode. The size is then coming from something structural, such as a fully embedded font, a repeated letterhead image, or file attachments.
How do I find what is making my PDF large?
Divide the file size by the page count. Over roughly 100 KB per page indicates images. Well under that with a large file indicates structural content such as embedded fonts or attachments.
What reduces a scanned PDF most?
Converting colour scans of monochrome documents to greyscale, then reducing resolution to no lower than 300 DPI. Both do more than JPEG quality reduction and cost less legibility.
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. Get it at 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.