BASALT · JOURNAL
Merging many large PDFs into one production
Combining a set of documents into one file is structurally simple and has a few properties worth knowing when the inputs are large.
The output is often smaller than the sum
Documents that share fonts, colour profiles or images can have those resources merged rather than duplicated. A set of 200 files produced from the same template can combine into something noticeably smaller than the total of its parts.
The opposite also happens. Files with no shared resources produce output that is the sum plus a little structural overhead.
Neither is a defect, and both are reasons to measure the result rather than predict it.
Preserve the boundaries
The most common regret after merging is losing track of where each source document began. A merge that writes a bookmark per source file preserves that at almost no cost, and it is the difference between a usable production and a two thousand page wall.
If the sources have their own bookmarks, those should be nested underneath rather than discarded.
Order matters and is easy to get wrong
Files sorted by name sort alphabetically, which puts document 10 before document 2. On a large set this is easy to miss and irritating to fix afterwards, since the merged file has to be rebuilt.
Check the order before running, not after.
Interaction with Bates numbering
Merging then numbering is usually the right order, because it produces a single continuous sequence with no continuation bookkeeping. Numbering then merging works too but requires the ranges to have been planned in advance.
What merging does not fix
A merged production inherits everything its sources carried: metadata, attachments, earlier revisions, hidden layers and any inadequate redaction. Combining files is not a sanitising step, and a document that was unsafe to send on its own is equally unsafe inside a larger file.
Sanitise after merging, verify after that.
Memory
Merging does not require holding the documents in memory. Pages are copied from source to destination, which is a streaming operation, so combining fifty 200 MB files is a disk exercise rather than a memory one. Budget space for the output alongside the inputs.
Frequently asked questions
Is a merged PDF smaller than the sum of its parts?
Sometimes. Documents that share fonts, colour profiles or images can have those resources combined rather than duplicated, so a set produced from one template merges smaller. Files with nothing in common produce output equal to the sum plus a little overhead.
How do I keep track of where each document starts after merging?
Write a bookmark per source file during the merge, and nest any bookmarks the sources already had underneath. Without that, a large merged production is very difficult to navigate.
Does merging PDFs remove hidden data?
No. The merged file inherits the metadata, attachments, earlier revisions and hidden layers of every source. Merging is not a sanitising step, so strip and verify after combining rather than before.
Does merging large PDFs use a lot of memory?
No, because pages are copied from source to destination rather than loaded. It is a disk operation, so budget free space for the output alongside the inputs.
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.