PDF (Portable Document Format) files can be combined into a single document using a process that concatenates their internal page streams. When you merge PDFs in your browser, the tool reads each file's page content, resources (fonts, images, embedded objects), and structure, then writes a new PDF file with all pages in your specified order. The resulting file is a standard PDF fully compatible with all PDF readers.
When to Merge PDFs
Common use cases include combining monthly bank statements into a single annual file, merging multiple chapters of a document, combining a cover letter with a resume and references into one attachment, or assembling multi-page scanned documents that were scanned page by page. Merged files are easier to share, archive, and organize than collections of individual files.
File Size Considerations
The merged PDF will be approximately the sum of the individual file sizes, sometimes slightly smaller if the tool deduplicates shared resources like embedded fonts. If your merged file is very large, consider compressing it afterward. PDFs with many high-resolution images are the biggest contributors to file size — text-heavy PDFs remain compact even when merged.
How many PDFs can I merge at once? ▾
There's no hard limit on the number of files. Practical limits depend on your device's available RAM — very large PDFs (100MB+) may be slow or cause browser memory issues on older devices. For most use cases (10–20 standard documents), merging is fast and reliable. If you hit issues, try merging in batches of 5–10 files first.
Is my data safe? Are files uploaded anywhere? ▾
Completely safe — your files never leave your device. The PDF merging happens entirely in your browser using the PDF-lib JavaScript library. Nothing is sent to any server. Unlike most online PDF tools (Smallpdf, ilovepdf, Adobe online), ToolStack processes everything locally. This means it works offline, your files stay private, and there are no size limits imposed by server policies.
Will the merged PDF retain bookmarks, hyperlinks and formatting? ▾
All page content, images, fonts and layout are fully preserved. Embedded hyperlinks within pages are retained. Document-level bookmarks (table of contents entries) may not carry over between files — this is a limitation of browser-based PDF processing. If bookmarks are critical, you'll need a desktop tool like Adobe Acrobat or PDF Arranger.