How do you combine or merge pdf(s) in Office 365 Business Premium or Windows?

Windows and Office do not ship with tools that can merge PDFs.

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Word 365 can edit some PDF files, if they are in the appropriate PDF format.

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There are free and paid tools on the internet you can use to merge and even edit PDF files

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5 Free PDF Editor Websites to Create, Fill, Annotate or Alter PDF Files                       2018 11 10               Mihir Patkar
https://www.makeuseof.com/tag/free-pdf-editor-websites-create-fill-annotate-alter-pdf-files/
PDF is the default file format for important documents. Whether it’s a form you need to fill or something you need others to read, these web apps will help you get everything done with a PDF, without ever installing any software.
You probably interact with PDFs many times a week, or even every day. Since these files look the same on any device, we still rely on PDFs heavily. There are a few tasks you typically want to do with them:
.  *  Fill or edit a PDF
.  *  Create a PDF form or document
.  *  Reduce file size (since PDFs can be much larger than a Word document)
.  *  Annotate a PDF or add notes that others can see
.  *  Split a PDF file
.  *  Merge multiple PDF files
.  *  Unlock a password protected PDF file

5 Free Tools to Edit PDF Files       2017 01 27
https://www.makeuseof.com/tag/tools-edit-pdf-files/
PDFs are one of the most common file types you’ll come across when using a computer. If you need to produce a high-quality print job, make a document available online, archive old data, or combine multiple formats (text, images, tables, etc.), there’s a good chance you’ll find yourself turning to the PDF format.
Despite their hundreds of benefits, they fall short when it comes to editing. Unless you have the luxury to use Adobe Acrobat Pro to manage your PDFs — the industry’s gold standard — it’s just not as easy to edit PDF documents as it is to edit Microsoft Office documents.
.  *   PDF Booklet-: Creating PDF booklets.
.  *  iLovePDF: Tweaking the formatting.
.  *  YACReader: Editing graphics.
.  *  Gaaiho Reader 4: Amazingly lightweight text editor.
.  *  Windows 10 Print to PDF        : Combining several images into a single PDF file.

This article is not on topic for your question, but you might find it interesting:

10 Tips to Do More With Your PDF Files on Google Drive                2019 08 29
https://www.makeuseof.com/tag/10-tips-pdf-files-google-drive/
Google Drive has come a long way from being an online-only, document viewer, and editor. Drive has some native features that make it an excellent PDF tool, but its compatible third-party Drive apps extend its usefulness and functionality.
.  *  Save to Google Drive From Chrome
.  *  Search With Optical Character Recognition (OCR)
.  *  Export Any Document to a PDF File
.  *  Scan Documents Into a PDF With the Mobile App
.  *  Annotate PDF Files With Google Drive Comments
.  *  Add, Delete, and Reorder Pages With DocHub
.  *  Fill Out and Sign PDFs With HelloSign or DocuSign
.  *  Merge Multiple PDFs With PDF Mergy
.  *  Separate Documents Into Pages With Split PDF
.  *  Save Storage Space With SmallPDF
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