About Merge PDF
Combining PDFs is one of those tasks that seems like it should be trivial and usually is not, because the obvious tools either want money, want an account, or want your document uploaded to a server you know nothing about. This does it on your own machine.
Select the PDFs you want to combine and they are merged in the order shown. Pages keep their original size and orientation, so mixing A4 and Letter, or portrait and landscape, produces a document where each page retains its own dimensions rather than being stretched to match its neighbours.
Why doing this locally matters more here than for most file types. The PDFs people merge are disproportionately contracts, invoices, tax returns, medical letters, scanned passports and signed agreements. Those are exactly the documents that should not be uploaded to an unknown server, and exactly the ones most often pasted into the first free tool a search returns. Here the merge happens in your browser: the file is read by code running on your own computer, and the page has no network permission to send it anywhere.
What is preserved. Page content, embedded fonts, images and vector graphics all carry across intact. This is a structural copy of each page rather than a re-render, so there is no quality loss and no re-compression — the pages in the output are the same pages, not pictures of them.
What is not. Interactive form fields, digital signatures, annotations and bookmarks are not carried over. Form data in particular is dropped, so if your source PDFs contain filled-in forms, flatten them in your PDF reader before merging. A digital signature is invalidated by any modification of the document, which is inherent to signing rather than a limitation here.
Encrypted files. A password-protected PDF cannot be merged; the tool detects this and says so rather than producing a broken output. Remove the password in your reader first.
Limits, and why they exist. Up to 30 files. The size limit adapts to your device: where the browser reports how much memory it can give a page, we use a share of that, so a desktop with plenty of headroom is allowed far more than a phone. Where the browser reports nothing, the limit is a conservative 800 MB. The ceiling is not arbitrary: a PDF writer has to hold the complete merged document in memory before it can hand you a file, so peak usage is roughly the combined size plus whichever source is being read. There is no way to stream a PDF out page by page — the format's cross-reference table is written last and refers to everything before it.
If you hit the limit, merging in halves and then merging the two results roughly halves the peak, because each half is written and released before the next begins. Beyond about a gigabyte, desktop software is the honest answer; we would rather say so than fail in a way that looks like a bug.
Questions
Are my PDFs uploaded?
No. The merge happens entirely in your browser, and the page is served with a Content Security Policy that prevents it from uploading your files. You can confirm this in your browser network tab.
Can I control the order?
Files are merged in the order they appear in the list after you select them. Select them in the order you want, or add them in batches.
Does merging reduce quality?
No. Pages are copied structurally rather than re-rendered, so there is no re-compression and no visible change.
Why did my form fields disappear?
Interactive form fields and annotations are not carried across. Flatten the form in your PDF reader before merging if you need the values kept.
It says my PDF is password-protected.
Encrypted PDFs cannot be merged without the password. Open it in your reader, remove the protection, and save a copy first.