What merging actually does to your file
Merging here is a copy operation, not a conversion. Each page object is lifted out of its original document and placed into a new one exactly as it was — the same content stream, the same embedded fonts, the same images at the same resolution. Nothing is re-encoded, so nothing is lost. The text in the merged file is still real text: you can select it, search it and copy it, and a screen reader can still read it.
That matters more than it sounds. Plenty of online tools merge by rendering every page to an image and stitching the pictures together. It works, it is easy to build, and it quietly destroys the document — the file balloons, the text stops being searchable, and anyone who needs a screen reader is locked out. If a merged PDF comes back three times larger than the sum of its parts, that is what happened to it.
Page order is the part most tools get wrong
Most mergers work at the file level: you order the documents, and the pages follow. That is fine when you are appending an appendix to a report. It is useless for the job people actually have, which is usually a scanner that produced odd pages in one file and even pages in another, or a contract where one page needs to move.
This studio works at the page level. Every page from every file lands in the same grid, numbered in output order, labelled with the file it came from. Drag one page, or select a run of pages with shift-click and move the whole block. Rotate the ones that came in sideways. Delete the cover sheet the scanner added. Then merge once, at the end, when the document already looks right.
Nothing is written to disk until you press the button, so every one of those moves is instant and reversible. Undo goes back thirty steps.
What it cannot do
Encrypted files are the hard stop. If a PDF asks for a password before it will open, it has to be unlocked first — that is what Unlock PDF is for, and it only works if you know the password. Anything that got around that would not be a tool, it would be an attack.
Interactive form fields and digital signatures also need care. Signatures certify a document as it was signed, so any change to it, including merging, invalidates them by design. Form fields with the same name across two files will collide. If either applies to your documents, flatten them first.
Everything else is bounded by your device rather than by a policy. There is no page cap, no daily limit and no size tier, because there is no server whose bill would go up. A few hundred pages merge in a couple of seconds on a laptop; on an old phone the same job takes longer and uses a lot of memory.