BASALT · JOURNAL
When a PDF will not open on your Mac
Before assuming the file is corrupt, work out whether it is the document or the application failing.
Check the file itself
qpdf --check document.pdfThis reads the structure without rendering, so it answers quickly even on very large files. If qpdf reports no problems, the document is sound and your viewer is the issue.
If the document is sound
The usual cause is size. An application that reads the whole file into memory before displaying anything fails on large scanned productions. Watch its memory in Activity Monitor while it opens: a figure climbing towards the file size means the document is being buffered, and your ceiling is available RAM.
Try Preview, which handles many files other applications refuse, and close other applications to free memory.
If the document is genuinely damaged
qpdf can often rebuild the cross reference table, which is the part that most commonly breaks in transit:
qpdf --replace-input --qdf --object-streams=disable broken.pdfWork on a copy. Repair is not always lossless and you want the original if it fails.
Common causes of real damage
An interrupted download, which is the most frequent. Re-download before doing anything else, and compare file sizes if the source reports one.
A file transferred as text rather than binary, usually through an old FTP workflow, which corrupts line endings throughout.
Storage failure, in which case other files are probably affected too.
Password prompts are not damage
A file asking for a password is working correctly. If you do not have it, no legitimate tool will open it, and a tool advertising that it can is either wrong or doing something you should not rely on.
Frequently asked questions
How do I know if a PDF is corrupt?
Run qpdf --check document.pdf, which reads the file structure without rendering it. If it reports no problems the document is sound and the application you were using is the limitation.
How do I repair a damaged PDF?
qpdf can rebuild the cross reference table, which is the part that usually breaks: qpdf --replace-input --qdf --object-streams=disable broken.pdf. Work on a copy, since repair is not always lossless.
Why does my large PDF fail to open?
Most often because the application reads the entire file into memory first, and the document is larger than the memory available. Watch memory in Activity Monitor: if it climbs to roughly the file size, that is the cause.
What causes PDF corruption?
Interrupted downloads are the most common, followed by files transferred as text rather than binary, and storage failures. Re-downloading fixes the first and is worth trying before anything else.
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.