Every tool here runs entirely inside your web browser. Your files are never uploaded to a server — there is no server to upload them to. This page explains, in plain terms, how that works and what each tool actually does to your PDF.
The core idea: your browser does the work
Most online PDF tools send your file to their servers, process it there, and send the result back. That means your document — which might be an ID, a contract, or a bank statement — sits on someone else's computer, even if only briefly.
CorePDFTools takes a different approach. When you open a tool, your browser downloads a small amount of processing code (once), and from then on it does all the work locally using your own device's processor and memory. The file you choose is read directly into the browser tab and never transmitted anywhere. You can even disconnect from the internet after the page loads, and most tools still work.
What runs under the hood
The tools are built on well-established open-source libraries that run as JavaScript and WebAssembly inside your browser:
pdf-lib — reading and writing PDFs
A PDF is essentially a container of objects: pages, text instructions, fonts, and images, all referenced by an internal table. The library we use reads that structure, lets us copy, reorder, rotate, or add to those objects, and writes a new, valid PDF back out. Because it manipulates the objects directly, operations like merging and splitting copy your pages exactly — nothing is re-rendered or degraded.
PDF.js — rendering pages to images
For tools that need to turn a page into a picture (PDF to JPG) or read a scanned page (OCR), we use the same rendering engine that powers PDF viewing in major browsers. It draws each page onto a canvas in your browser, which we can then save as an image.
Tesseract — reading text from scans (OCR)
Optical character recognition analyses the image of a scanned page and recognises the characters, turning a picture of text into real, selectable text. This runs in your browser too, so even sensitive scanned documents never leave your device.
How each tool actually works
Merge, Split, Extract, Organize
These copy your existing pages into a new document in the order you choose. Because the page objects are copied as-is, there is no quality loss and no re-compression — the output pages are identical to the originals.
Rotate
Rotation is stored as a property on each page. We set that property and save, so the rotation becomes a permanent part of the file rather than just a temporary view setting.
Compress
This is the most involved tool. A large PDF is almost always large because of embedded images, not text. So instead of flattening your pages into pictures (which would blur text and often make files bigger), our compressor locates the embedded JPEG images inside the PDF, decodes each one, downsamples it to a sensible resolution, re-encodes it at your chosen quality, and writes it back in place. Your text and vector graphics are left completely untouched, so they stay sharp and selectable. As a safeguard, the tool compares the result to your original and never hands back a file larger than what you started with.
Compress to a target size
For upload limits like 100KB, the tool applies progressively stronger settings — reducing image resolution and quality step by step — until the file fits under your chosen size, while keeping text legible.
JPG to PDF / PDF to JPG
Images are embedded as pages (JPG to PDF), or each page is rendered to a canvas and saved as an image (PDF to JPG). Both happen locally.
Protect
Password protection encrypts the PDF's contents using the password you provide, so the file can only be opened by someone who knows it. The encryption happens on your device; neither the file nor the password is uploaded.
Sign
You draw a signature on a canvas, position it on the page, and we embed that signature image into the PDF at the spot you chose.
Watermark, Page numbers
Text is drawn as an overlay across each page and saved into the file.
OCR
Each page is rendered to an image, then the recognition engine reads the characters and outputs the text — all in your browser.
The honest limitations
Running everything in the browser is great for privacy, but it has trade-offs we'd rather be upfront about:
Very large or many-page files depend on your device's memory, so they can be slow — especially on older phones. Compression works best on scanned and image-heavy PDFs; a document that is already mostly text has little to compress. And some specialised image formats inside PDFs are skipped during compression to avoid damaging them. For the everyday tasks most people need, though, local processing is fast, private, and free.