How to Split PDF By File Size
Email attachment limits, upload form restrictions, and storage quotas often require splitting a large PDF into smaller chunks that each fit under a maximum file size. Splitting by page count alone doesn't guarantee size — a 10-page PDF with high-res photos on pages 3-5 may produce unevenly sized parts. SublimePDF's size-based split divides a PDF into parts that each stay under your specified limit.
Follow the step-by-step instructions below, then use the free tool directly — no registration or download required.
Open Tool →How to Split PDF By File Size — Step by Step
Upload your PDF
Open the split tool or POST to /api/v1/split with your large PDF file. The tool analyzes the document structure and page-level size distribution.
Set the maximum file size per part
Specify the size limit: { "splitMode": "size", "maxSizeMB": 10 }. The tool calculates the optimal page groupings to keep each output file under this threshold. Common limits: 10 MB (email), 25 MB (Gmail), 5 MB (form uploads).
Review the proposed split plan
The API returns a preview of how pages will be distributed: { "parts": [{ "pages": "1-15", "estimatedSize": "9.2 MB" }, { "pages": "16-22", "estimatedSize": "8.7 MB" }] }. Review to ensure logical content isn't awkwardly divided.
Adjust split points if needed
If the automatic split breaks content in an undesirable place (mid-chapter, mid-table), override with manual page boundaries: { "splitMode": "size", "maxSizeMB": 10, "preferBreakAfter": [15, 30, 45] }. The tool uses your preferred break points when they fit within the size constraint.
Download the split files
The API returns the split files as a ZIP archive. Each file is named sequentially (document_part1.pdf, document_part2.pdf) and includes the original's metadata and bookmarks for pages in that part.
Pro Tips
- 💡 Account for overhead — each split part includes PDF metadata, fonts, and shared resources. A 50 MB PDF split into 10 MB parts may produce 6 parts rather than 5 due to this per-file overhead.
- 💡 For email distribution, set maxSizeMB to 8 rather than 10 to leave room for email encoding overhead (base64 encoding increases attachment size by ~33%).
- 💡 If specific pages are disproportionately large (full-page photos or charts), consider compressing the PDF first to reduce per-page sizes before splitting.
- 💡 Use the 'preferBreakAfter' option to split at chapter or section boundaries for a better reading experience when recipients receive the parts sequentially.
Privacy & Security
All processing happens directly in your browser. Your files are never uploaded to any server — they remain on your device throughout the entire process. SublimePDF uses WebAssembly technology for fast, secure, client-side processing.
Works Everywhere
This tool works on any modern browser — Chrome, Firefox, Safari, or Edge — on desktop, tablet, or mobile. No software to install. PDF is an open ISO standard supported by all major platforms.
How to Split PDF By File Size — FAQ
How does size-based splitting differ from page-based splitting?
Can a single page exceed the maximum size limit?
Are bookmarks preserved in split files?
Can I recombine the parts later?
Related Guides
How to Split PDF Pages
Need to extract specific pages from a large PDF or split a document into separate files? SublimePDF's Split PDF tool makes it easy — select your pages and download instantly.
How to Split PDF By Bookmarks
Splitting a PDF by its bookmarks is the most efficient way to break a large document into logical sections — each bookmark becomes a separate file. This is invaluable for distributing individual chapters of a book, separating sections of a policy manual for different departments, or extracting specific parts of a lengthy report. SublimePDF reads the bookmark structure and splits at each top-level bookmark automatically.
How to Split PDF Every N Pages
Splitting a large PDF at regular intervals is invaluable when you have a combined document that follows a repeating structure — such as monthly reports where each report is exactly 4 pages, multi-student grade sheets with 2 pages per student, or batch-scanned forms with a fixed page count. Instead of manually selecting page ranges, splitting every N pages automates the process and produces uniformly sized output files in seconds.