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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.

PDF Files: the Basics

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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 but Adobe's own Acrobat Reader is probably the most popular.  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.  

Some PDF files are text, images, and graphics that are so indexed within the file that they can be extensively edited in Adobe Acrobat.  Others are crude scans that consist of a single image - like a photocopy or photograph - of something that happens to be saved within a PDF file.  With the latter, Adobe's tools may be able to do little or no editing, besides perhaps crude text overlay or annotations.  What Acrobat can do with a scanned document depends on the quality and condition of the paper original, and the circumstances of it's scanning.  

Scanning

Creating a searchable (and possibly editable PDF) from a scan

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