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So you're adding content that you create into your PDF you also need a word processor, such as Microsoft Word, or Google Docs.  The files you wish to compile into a single PDF might be generated with various apps, such as AutoCAD, Excel, Google Drawings, or Canva, and they might be in formats such as .xlsx, .jpg, or Google Slides.  And you may include files that are in PDF format already, within your bigger PDF.  These may be files that were always digital, having been saved as PDFs in creation software (such as Word or Google Docs.)  Or they can be scans of paper documents, that are essentially images saved in PDF format.  Adobe Acrobat can combine all of these into a single, navigable PDF document readable by anyone with a PDF reader application.  

PDF Files: the Basics

A PDF ("Portable Document Format") file offers a consistent format across PC or mobile devices that have a PDF reader.  There are many free PDF readers, including but not limited to Acrobat Reader, Adobe's free version of Acrobat.  A PDF file is not designed to be easily or extensively edited; it's a published product, rather than a draft.  Create your content in tools like Microsoft Word, Excel, Powerpoint, or Google Docs, Sheets, Slides, or Drawings.  Then, within Adobe Acrobat, you can add these together into a PDF file, and do final arrangement and organization, together with some minor edits, if needed. You can also scan in paper copies of documents, but born-digital documents are always best when working within Acrobat and PDFs. Major editing, reformatting, or adding additional content is best done in the original document files, such as .docx, or Google Docs, rather than in Acrobat.  Since it isn't built for extensive content creation, Acrobat's tools can be awkward and inefficient for major changes to the content within a PDF.  

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Combining PDF Files 

Here's a video tutorial of the process  Below is a text/screenshot tutorial as well.

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Once you are satisfied with their order, click "Combine," and Adobe will generate a single PDF file using these documents.  

Arranging your Contents

Once you have files assembled into a PDF file, you can rearrange them in several ways.  Click the "Organize Pages" icon on the right toolbar.  (Or, click Tools → Organize Pages.)

Adding Another File

If you forgot to add a file, you can still do so after having created the PDF file.  Recognize, however, that if you have many additions, and you have already created a Table of Contents within one of the pages in the file, there are many more steps involved and you may be better off simply creating a new Table of Contents page, and recompiling a new PDF file.

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You can then find and add the file or files from your Hard Drive.  

Rearranging Files

On the Organize Pages screen, you can drag and drop pages into a different order.  If you hold down the SHIFT key, you can click/select multiple pages, which can make moving them easier.

Rotating Pages

Rotate pages by clicking on the page to highlight it.  A small menu appears; click the circle-arrow icons to rotate the page image in either direction.  

Organization via Bookmarks and Thumbnails

You may stop at the above step, if you're happy with what you have: a set of various documents combined into one PDF File.  Plus, Adobe's bookmarks feature may be all that you wish to use in adding navigation to your PDF file. 

Bookmarks  

PDFs can contain a table of contents or organization that exists separate of any page within the document.  In Acrobat or Acrobat reader, this typically appears on the lefthand side when the Bookmarks tool is made visible.  These bookmarks are often also visible in other PDF readers. 

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Thumbnails

PDFs can contain a set of small "thumbnail" images of each page that together form a quick-navigation tool.  In Acrobat or Acrobat reader, this typically appears on the lefthand side when the Page Thumbnails tool is made visible.  This tool is probably less valuable for the reader, since the author can better recognize pages represented by the thumbnails, and only pages, rather than headings, are navigation choices here.

Page Numbers

You can add page numbers to your combined PDF file, and although they will not neatly replace page numbers visible in the original documents added to the PDF file, they still can be helpful for readers to navigate the whole.  Click the Edit Icon (or click Tools → Edit).  In the toolset at the top, click "Header & Footer," and from the dropdown menu, click "Add."  You can add all sorts of information on the header, using the various text boxes for "Left Header Text, Center Header Text," and so on.  Place your cursor in "Right Header Text," and click the "Insert Page Number" button to simply add a page number set to the PDF file. 

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If you plan to add front matter to your document that is not paginated, you may wish to wait until that's done before creating page numbers.

Table of Contents

You may wish to go a step further, and create a Table of Contents as featured on a page or pages, presumably at the beginning of your PDF file.  Although not a toolset built into Acrobat, you can create a dedicated Table of Contents that includes just page numbers, or are actively linked to your content.  Unlike Bookmarks, the Table of Contents is a page within your portfolio, wherein the items in the Table list are hyperlinked to pages within the PDF.  When your readers click on items in the table of contents, they are taken to the corresponding page.  This presents a professional-looking product, and offers the option to create a document that is easily navigated if printed out, too. 

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