BASALT · JOURNAL
Why Preview struggles with large scanned PDFs
Preview is a good viewer and it is what most Mac users reach for. On large scanned documents it becomes slow, and the reasons are worth understanding because they are about design choices rather than defects.
Thumbnails
Preview shows a page thumbnail sidebar, and thumbnails require rendering pages. On a thousand page scanned document that is a thousand image decodes, and while the work is spread out rather than done all at once, it is real and it competes with whatever you are trying to do.
Turning the sidebar off helps noticeably on large files.
Rendering photographs
Every page of a scan is a JPEG that has to be decoded before it can be drawn. A text page is instructions and draws quickly; an image page is a decode. Scrolling through a scanned production is therefore a sequence of image decodes rather than a sequence of cheap draws.
This is inherent to the document rather than to Preview.
The redaction problem is separate and more serious
Preview's annotation tools can draw a black rectangle over a page. That rectangle is an annotation drawn on top, and the text or image underneath remains in the file. Selecting the region and copying, or deleting the annotation in any PDF editor, recovers what was supposedly removed.
This is not a performance issue but it is the reason not to use Preview for redaction, and it is worth stating plainly because the visual result looks identical to a real redaction. The page renders the same either way, which is exactly what makes the mistake so easy to make and so hard to catch by looking.
What Preview is genuinely good for
Reading, quick page rotation, simple reordering, signing, and exporting. All of these are honest operations with no hidden failure mode.
When to use something else
When the document is large enough that the thumbnail rendering gets in the way, when you need text search across a scanned production, when you need Bates numbering or a privilege log, and always when you need to remove content rather than cover it.
The last one is not about capability or speed. It is about whether the material is still in the file afterwards.
Frequently asked questions
Why is Preview slow with large PDFs?
Mostly the thumbnail sidebar, which requires every page to be rendered, and the fact that scanned pages are images needing decoding rather than instructions that draw quickly. Hiding the sidebar helps noticeably on large scanned documents.
Can Preview redact a PDF?
No. Preview can draw a black rectangle over a page, but that is an annotation drawn on top and the content underneath stays in the file. Copying the region or deleting the annotation recovers the material, so it should not be used to remove sensitive content.
Does a black box in Preview remove the text?
No. The page renders as though the text were gone while the text remains in the content stream, which is why the mistake is so easy to make: the visual result is identical to a genuine redaction.
What is Preview good for?
Reading, rotating, reordering pages, signing and exporting. These are straightforward operations with no hidden failure mode. Search across scans, Bates numbering and genuine redaction need other tools.
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.