Invented in the early 1990s by Adobe, PDF (short for Portable Document Format) has grown into a staple of the computer age. The format became an open standard in 2008.
PDFs were developed as a way to share highly compressed documents online, independent of applications, hardware, or operating systems. A PDF includes the complete file information such as text, fonts and images necessary to display a digital file, meaning everyone will see it the same.
Most documents do not include all this information; think of a Microsoft Word document (.doc or .docx). If you and I were to share a Word doc between us, we need to use fonts that both us of have installed on our own computers or else each of us will view the document differently. When there is a discrepancy, the missing font is replaced–in many programs completely unbeknownst to you–and you see something completely different than what was originally created.
Because of the PDF's all-in-one format and its compression capabilities, it became extremely useful when sharing files that needed to look a certain way each time. Whether outputting for print or maintaining branding guidelines, the PDF provided a way to ensure files were uniform and has become the standard for sharing documents on the web.
PDFs can be a great way to provide your customers or visitors with convenient, downloadable content. Whether it's a copy of your menu, an instruction manual for a product, or an employment application, PDFs allow for quick download because of their compressed size.
Because the PDF format (that's like saying "ATM machine, isn't it) is a standard, they can be created in multiple applications. Most computers even give you the option to "Print as PDF" so that you can create a file from any printable document, be it an image or text. Rather than physically print order confirmations like retailers always tell you to do, you can save some trees and save them all as PDFs. (or ignore it completely since you'll probably be getting something in your email anyways!)
If created correctly, PDFs can even contribute to your SEO efforts. A file that has been scanned in or taken as an image won't work in these circumstances. Most files that are created using a text program, however (like Word, InDesign, etc) or saved from a webpage will have selectable text.
Try this test - If you can select text with your cursor, it's readable and crawlable by Google. This makes the content in those PDFs a valuable addition to your SEO. Optimizing PDFs can go far beyond the text, however. Alex Fusman has a great article on Moz about all of the aspects to keep in mind when creating and prepping your PDFs for the web.
As our Search Engine Optimizer, Rich always says: More content on your site creates a bigger footprint. A bigger footprint is easier to find, so the more content the better. While PDFs can boost that footprint, there are downsides to relying so heavily on a file, rather than your site's HTML content.
When your PDF is returned as a result on a SERP (search engine result page) and a user clicks through, they only see the file. Your site's branding, styling, and navigation are all gone, all of which the likelihood that the user will bounce once they have looked at it. A savvy user can identify your main site from looking at the URL and navigating there, but many times the back button is the easiest option.
A solution to this dilemma is to migrate the important content onto your site as crawlable HTML, while keeping the PDF as a resource to download. (If it's not a document that someone is likely to download, then why is it a PDF on your site in the first place?) Using canonical tags on the site pages, you can alert Google and other search engines to the fact that while the page and the PDF contain duplicate content, only the page should be 'counted.'
Canonical tags take care of duplicate content issues related to SEO, but you need to stay on your toes. If for any reason information needs to be updated (like prices on your menu) you'd better remember to update BOTH the page and the PDF or you'll have some confused customers!
As long as you are aware of the issues, there really isn't an ugly side to PDFs. They have undoubtedly changed the digital landscape in regards to how and what we can share with each other.
If you need a PDF reader, you can download Adobe Reader for free. PDF is an open format, so there are many more options out there, but Adobe Reader is free. Why not get the program from the people who created the PDF in the first place?