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Merge scanned documents into one PDF

Merging scans is the same operation as merging any other PDFs, with one extra problem at the end: the result is large. Join the documents first, in one pass, then compress the finished file once. Doing it in that order means the images are re-encoded a single time instead of repeatedly.

Merge PDF

Merge first, compress second

Open the merge tool, add every scanned document, put them in order and merge. Then take the result to the compression tool and name the size you need.

The reverse order costs quality for no benefit. Compressing each part and then merging puts every page through image encoding, then copies the already encoded images into a new document, and you end up guessing at per-file sizes that add up to something unpredictable. One compression pass on the finished document targets the number you actually need.

Why the merged file is so big

A scan is a picture of a page. Even at a modest resolution, a picture of a page carries far more data than the same page as text, and a document feeder set to colour makes it several times worse again. Ten scanned documents of twenty pages each is easily a few hundred megabytes.

Merging does not add to that and does not take anything away. Pages are copied as they are, so the merged file is close to the sum of its parts.

How far a scan will compress

Scans are the best case. On the public bench run, documents of the scan type met their requested size on 88.9% of cases, against 20.0% for text documents, because a photographed page has slack in it that a page of real text does not.

The engine works to a byte target rather than a quality slider. You name a size, several methods run against it, and each result is read back and checked before it is offered. Across the whole run the median file came out 59.2% smaller than the source and no output was ever larger than its input.

What the merge keeps and what it clears

Every page keeps its content, its width and its height, so a stack that mixes A4 and Letter stays mixed. The merged file is reopened and compared page by page against receipts taken from the sources before it is handed over, and a mismatch means the merge is refused rather than downloaded.

Document properties are cleared, so the merged PDF carries no title, author or dates from any of the scans. Bookmarks, form fields, page labels and attachments do not survive either, and the tool names which sources carry them before it runs.

A scan that has been through text recognition keeps that text layer, because the text is part of the page. Its accuracy is whatever the recognition produced, which merging neither improves nor damages.

Before you merge

Check that every part is the document you think it is. Scanner output named scan0042 tells you nothing, and finding a wrong file after merging means doing the whole job again.

If the parts arrived as photographs rather than PDFs, the multi-page scanning guide covers turning them into pages first. The merge hub lists the rest of the cluster.

Getting the order right across separate scans

Scanned files carry no clue about where they belong. A folder of scan0031 through scan0074 is in the order the sheets went through the feeder, which is not page order if the stack jammed, if somebody scanned the reverse sides in a second pass, or if one document was rescanned.

Open the first page of each file before you merge and write the real sequence down. Then set that order in the tool by dragging, and check the page count shown beside each file against what you expect. A file holding twelve pages where you expected one is worth catching before you merge rather than after.

Questions people ask

Should I clean up the pages before or after merging?
Delete and rotate first, then merge. Fixing a sideways or blank page in one small file is quicker than hunting for it inside a long merged document.
Should I compress scanned files before or after merging them?
After. Merge everything into one document, then compress once against the size you actually need. Compressing the parts first puts the images through encoding twice and makes the final size a guess.
Does merging make scanned pages look worse?
No. Pages are copied without being re-rendered, so a merged scan looks exactly like its source. Quality only changes if you compress afterwards, and then only as far as your size target requires.
How much smaller can a merged scan get?
On the public bench run, scan type documents met their requested size on 88.9% of cases and the median result across all types was 59.2% smaller than the source. Your file depends on its own content.
Does the text layer from OCR survive?
Yes. Recognised text is part of the page content, so it is copied with the page and stays selectable and searchable. Its accuracy is whatever the recognition produced.

Do it now

The tool runs in this browser. Your file never leaves the machine, and the result is checked before you download it.

Merge PDF

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