How to Split a PDF by File Size

Splitting a PDF by file size means breaking one large document into several smaller PDFs, where each output file stays at or under a size limit you set, like the 25MB cap Gmail enforces on attachments or the 10MB limit a lot of web upload forms use. Unlike splitting by page number, you don't need to know how many pages fit under the limit; you just pick the size and let the tool work out the grouping. LockerPDF's Split PDF by File Size tool does this directly in your browser, so the file itself is never uploaded anywhere while it happens.

Why split by size instead of by page range

A PDF's file size depends on what's inside each page, not how many pages it has. A 40-page PDF of plain text might be 500KB total, while a 5-page PDF full of high-resolution scans can be 30MB on its own. That means if your goal is "get every file under 10MB so email will accept it," picking page ranges by hand means guessing, checking the resulting size, and re-splitting if you guessed wrong.

Splitting by file size skips that guesswork. You tell the tool the ceiling, and it figures out exactly where to break the document so every output file lands at or under that number, checking the actual size of each candidate file rather than estimating it.

How the automatic grouping works

The tool reads your PDF's pages in order and builds each output file by adding pages one at a time, starting from page 1. After adding a page, it checks the real size of that file as it would actually be saved. If adding the next page would push the file over your chosen limit, that page gets held back to start the next file instead. This repeats until every page has been placed into a file.

Two things follow from that logic. First, pages are never reordered: the first output file always starts at page 1 and files are produced in order, so the document's structure stays intact across the split. Second, if a single page's own size is already bigger than your chosen limit, for example one page holding a very high-resolution image, that page still ships on its own in its own file, since a page can't be split further. Every other output file still stays within your limit.

How to split a PDF by file size, step by step

1. Go to the Split PDF by File Size tool and drag your file into the dropzone, or click to browse for it.
2. Once it loads, you'll see the page count and file size, plus a preview of the first page.
3. Choose a maximum size per output file: pick a preset (5, 10, or 25 MB) or enter a custom value in the field next to the presets.
4. Click "Split by Size." The tool groups your pages automatically so each resulting file stays within that limit.
5. Download the result. If your document fits entirely within one output file, you get a single PDF. If it needed to be broken into more than one file, you get a ZIP archive containing part-1.pdf, part-2.pdf, and so on.

Common reasons to split a PDF by size

The most common trigger is an email attachment limit. Most providers cap attachments somewhere between 10MB and 25MB, and a scanned contract, a photo-heavy report, or a bundle of merged documents can easily exceed that in a single file. Rather than manually splitting the document and checking each piece's size in your file explorer, you set the limit once and get files that are already guaranteed to fit.

Upload limits on web forms and portals are another common case: job application systems, government e-filing sites, and client portals often enforce their own size caps, sometimes stricter than email. And even outside of a hard limit, breaking an unwieldy PDF into a handful of more manageable pieces, say by chapter-sized chunks, can just make a document easier to send, store, or review a section of.

How this is different from Split PDF

LockerPDF's Split PDF tool splits a document by page ranges you specify yourself, like 1-3,5,7-9, or by breaking every page into its own file. It has no concept of file size at all; it only cares about which pages you asked for. Split PDF by File Size works the opposite way: it has no concept of page ranges, only of the size ceiling you set, and it figures out the page groupings on its own so the result fits. If you already know exactly which pages you want pulled out, use Split PDF. If your goal is "get files under X megabytes" and you don't want to work out the page math, use this tool instead.

When splitting isn't what you actually need

Splitting by size produces smaller files, but it doesn't make the underlying content any smaller; a 30MB document split into three 10MB pieces is still 30MB of total content spread across three files. If what you actually want is one file with a genuinely smaller total size, for example because your images are far higher resolution than the document needs, Compress PDF is the right tool instead. It re-renders each page to reduce file size, which is a real tradeoff worth knowing: compressed pages become fixed images, so text is no longer selectable or searchable afterward. Splitting by size, by contrast, doesn't touch page content at all; it only reorganizes existing pages into separate files.

Why this doesn't require uploading your file

Both reading a PDF's page structure and measuring the byte size of a candidate output file are things a JavaScript library can do inside a browser tab, without needing a server to do the work. LockerPDF's Split PDF by File Size tool uses pdf-lib to build each candidate file locally, check its real size, and either keep growing it or start a new one, entirely in memory on your device. You can confirm this yourself: open your browser's developer tools (F12), go to the Network tab, and run a split. You won't see your document's bytes going out over the network.

FAQ

Are my files uploaded anywhere when I split by size?

No. The entire process runs in your browser using JavaScript: your PDF is read from disk, grouped into output files in memory, and handed back to you for download without being sent to a server at any point.

How does the tool know where to split the pages?

It adds pages to the current output file one at a time, checking the file's real saved size after each addition. Once adding the next page would push that file over your chosen limit, it starts a new file with that page instead. Pages always stay in their original order.

What if one page alone is bigger than my size limit?

That page still ships in its own file by itself, since a single page can't be split into pieces. Every other output file in the result still stays at or under the limit you chose.

What format do I get back?

If your document fits entirely within one output file at your chosen size, you get a single PDF download. If it needed more than one file, you get a ZIP archive containing part-1.pdf, part-2.pdf, and so on in order.

Does splitting by size reduce quality or compress the pages?

No. Pages are copied as-is into each output file; nothing is rasterized, compressed, or watermarked. If you need the total content to actually take up less space, not just be divided into smaller pieces, use Compress PDF instead.