Merging PDF files is one of the most common document tasks — combining chapters, attaching supporting documents to an application, or bundling invoices for accounting. Most search results point to tools that ask you to sign up or send your files to a server. There's a better way.
Why most free PDF mergers aren't really free
"Free" often means one of three things online: ad-supported with a file size cap, a free tier that requires email sign-up, or a trial that limits how many files you can merge before demanding payment.
Beyond the cost, there's the upload problem. Most PDF merger sites work like this:
- 1.Your files are uploaded to their servers — potentially across multiple data centres.
- 2.The server combines them.
- 3.The result sits on their infrastructure until it expires — typically 1–24 hours.
If the PDFs you're merging contain contracts, payslips, medical documents, or personal information, this is a meaningful privacy risk — even if the service is reputable.
How browser-based merging works
Modern browsers can merge PDF files entirely locally using JavaScript. The library that makes this possible is pdf-lib — a well-maintained open-source library that reads, modifies, and writes PDF files entirely in JavaScript, with no server required.
When you merge PDFs using LocalDF's merger:
- pdf-lib reads each PDF into memory using the File API
- A new, empty PDF document is created in memory
- Pages are copied from each source document into the new PDF, in your chosen order
- The merged document is offered as a download — direct from browser memory to your disk
No bytes of your files cross the network. The merge runs in a Web Worker (a background thread), so the browser stays responsive even when processing large documents.
Step-by-step guide
- 1
Open the PDF merger
Go to localdf.app/merge in Chrome, Firefox, Safari, or Edge. No sign-up, no extension, no app to install.
- 2
Drop your files
Drag and drop multiple PDF files onto the drop zone, or click to open a file browser. You can add files in batches — any files you drop are added to the list.
- 3
Arrange the order
Files appear in a numbered list. Use the ↑ ↓ arrows to reorder them. The pages will appear in your merged PDF in the order shown.
- 4
Merge
Click the Merge button. A progress bar shows as pdf-lib copies pages from each source document. For typical files (10–20 MB total), this takes 1–3 seconds.
- 5
Download (and optionally compress)
Click Download Merged PDF to save the result. If the merged file is large, you can also click Compress then Download — this runs the merged result through Ghostscript locally, often reducing the size by 40–70%.
Tips for best results
Check orientation before merging
If you're combining portrait and landscape pages, they'll be mixed in the output — which is usually intentional. If not, use the Fix Page Size tool to standardise orientation first.
Fonts and formatting are preserved exactly
pdf-lib copies pages without re-encoding content, so all text, images, and formatting appear identically in the merged output. There's no re-rendering step that could shift layouts.
Password-protected PDFs
Lightly encrypted PDFs (open-with-no-password, edit-protected) usually merge without issues. Heavily encrypted files may fail. Remove password protection first if you encounter an error.
Frequently asked questions
How many PDFs can I merge at once?
There's no hard limit. You're limited only by your device's available memory. Merging 10–20 typical PDFs (5–10 MB each) works smoothly on any modern device.
Will merging reduce quality?
No. pdf-lib copies pages without re-encoding any content. Text, images, and vector graphics are copied bit-for-bit from the source documents.
Does it work on iPhone or Android?
Yes. The merger works on Safari (iOS), Chrome (Android), and any other modern mobile browser.
Can I merge PDFs with different page sizes?
Yes — each page retains its original dimensions. If you want a uniform size throughout, use the Fix Page Size tool after merging.
Merge your PDFs now — free, no account
LocalDF merges PDFs using pdf-lib, entirely in your browser. No upload, no sign-up, no file size limit per file.
Merge PDFs now →