BASALT · JOURNAL
Splitting a large PDF by size rather than page count
Most splitting tools divide by page count, which works well for text documents and badly for scans. In a scanned production, page sizes vary enormously: a blank page is a few kilobytes and a dense colour page can be several megabytes, so a hundred page split can produce chunks that differ by an order of magnitude.
When the reason for splitting is an upload limit or an email attachment cap, page count is the wrong unit entirely.
Splitting by measured size
The reliable method is to accumulate pages until the output would exceed a limit, then start a new file. The important word is measured: predicting output size from input size is unreliable, because pages carry shared resources such as fonts and images that may be duplicated or dropped depending on where the split falls.
A tool that measures rather than estimates will never hand you a chunk that exceeds the limit you set, which is the entire point of setting one.
Splitting is lossless, compression is not
This matters more than it sounds. Splitting copies page objects as they are, with no decoding and no re-encoding, so the content of each chunk is byte for byte what it was in the original.
Compression is lossy on images. Every JPEG generation degrades a little, and a scanned production that has been through several rounds develops artefacts around text that were not in the original document.
So when a file is too large to send, split it. Reach for compression only when the recipient needs one file and the size must genuinely come down, and then compress a copy.
Where to split
On document boundaries, if you know where they are. A production is usually a stack of separate documents concatenated, and splitting at arbitrary page counts cuts them in half.
If the file has bookmarks, they often mark those boundaries. If it has Bates numbers, the ranges do. Splitting blindly every hundred pages is the last resort, not the default.
After splitting
Check the total. The sum of the chunks should account for every page of the original, and it is worth confirming rather than assuming, because an off by one at a boundary is easy to make and hard to notice later.
Frequently asked questions
How do I split a PDF into files under a size limit?
Accumulate pages until the next one would exceed the limit, then start a new file, measuring the real output size rather than estimating it. Page count is unreliable for this because scanned pages vary hugely in size.
Does splitting a PDF reduce quality?
No. Splitting copies page objects without decoding or re-encoding, so each chunk contains exactly the original content. Quality loss comes from compression or conversion, not from splitting.
Should I split or compress a large PDF?
Split when the constraint is a transfer limit, since it is lossless. Compress only when the recipient genuinely needs a single smaller file, and compress a copy, because image compression is lossy and repeated generations degrade scanned text.
Where should a large production be split?
On document boundaries where possible, using bookmarks or Bates ranges to find them. Splitting at fixed page counts cuts individual documents in half, which makes the output harder to use.
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.