How to merge PDF files
You have three PDFs that need to be one PDF. A cover letter, a resume, and a reference list. Or five chapters of a report that were exported separately. Or a stack of scanned receipts for an expense claim. Combining them should be trivial, and on a Mac it is. On Windows it takes a bit more work, but it is still free.
On Mac: Preview does it natively
Apple built PDF merging directly into Preview, and it works better than most people realise.
Open the first PDF in Preview. Go to View and select Thumbnails to show the page sidebar. Now drag the second PDF file from Finder into the thumbnail sidebar at the position where you want it inserted. The pages appear immediately. Repeat for any additional files. When the order is right, go to File, Export as PDF, and save the combined document.
You can also rearrange individual pages by dragging thumbnails up and down, and delete pages you do not need by selecting a thumbnail and pressing Delete. Preview handles this smoothly even with large files — a hundred-page merged document is no problem.
One thing to watch: use "Export as PDF" rather than "Save." If you just hit Save, Preview will modify the first PDF you opened rather than creating a new combined file. Export creates a fresh copy and leaves the originals untouched.
On Windows: PDFtk Free or PDFsam
Windows does not have a built-in PDF merger, but two free tools handle it well.
PDFsam Basic is an open-source application with a clean interface. Install it, open the Merge module, drag in your PDF files, arrange the order, and click Run. The output is a single combined PDF. PDFsam runs locally — no internet connection needed, no file uploads. It also supports splitting PDFs, rotating pages, and extracting page ranges.
PDFtk Free is an older tool with a simpler interface. It does the same job — drag files in, set the order, click Create PDF. It has been around since 2004 and is widely trusted, though the interface looks its age.
Both tools are genuinely free (not trial versions) and process files locally on your machine.
On Linux: command line with pdftk or pdfunite
Most Linux distributions include or can install pdftk or pdfunite (part of the poppler-utils package).
With pdftk: pdftk file1.pdf file2.pdf file3.pdf cat output merged.pdf
With pdfunite: pdfunite file1.pdf file2.pdf file3.pdf merged.pdf
Both commands take a list of input files and produce a single output. Simple, fast, and scriptable for batch processing.
Online mergers
Smallpdf, ILovePDF, Adobe Acrobat Online, and others all offer browser-based PDF merging. Upload your files, arrange the order, download the result. They work, and for non-sensitive documents they are convenient.
The trade-off is the same as with any online file tool. Your PDFs are uploaded to the operator's server for processing. If the documents contain personal, financial, or confidential information, you are trusting the operator to handle them properly. For a set of product spec sheets, that is fine. For a set of medical records or legal documents, use a local tool.
There is also a practical limitation: most free online mergers cap the total file size at 50-100 MB. If you are combining large scanned documents, you may hit that limit.
What happens to metadata when you merge
When you combine PDFs, the resulting file gets new metadata — a new creation date, and the software used for merging is recorded. The metadata from the individual source files (author names, creation dates, document titles) is typically discarded or overwritten.
This is usually fine, but if you need specific metadata in the output — a particular author name or title — you may need to set it after merging. Adobe Acrobat lets you edit PDF metadata directly. On Mac, some metadata can be edited through Preview's inspector panel. For a metadata-free output, tools like MetaScrub's PDF metadata remover can strip everything clean after you merge.
Page size mismatches
If you are merging PDFs with different page sizes — say an A4 report with a Letter-sized appendix — the merged file keeps each page at its original size. The pages do not get rescaled to match. This is usually what you want, but it can look odd when scrolling through the document, and it may cause issues when printing double-sided.
If consistent page size matters, the cleanest approach is to "print" the final merged PDF to a new PDF with a fixed page size. On Mac, use the print dialog's Paper Size setting. On Windows, "Microsoft Print to PDF" lets you set the output size. This re-renders every page at the chosen dimensions.
The quick answer
Mac: open in Preview, drag files into the sidebar, Export as PDF. Windows: install PDFsam Basic, drag and merge. Linux: pdfunite in the terminal. Sensitive documents: always use a local tool, never an online one.