Adobe came out with Acrobat software in 1993, designed to standardize the process. Suddenly, companies with different software and fonts could transport files to each other as WYSIWYG (What You See Is What You Get) meaning that what the creator saw on the screen was what the recipient also saw, regardless of software. This was pretty innovative thinking at the time, and very quickly became the standard for delivery of print files.
As the internet became widely embraced, many companies with annual reports decided to take their print-ready PDF and publish it online. This was a matter of convenience and timeliness for shareholders, to be able to download it this way and not wait for the printed issue; it was convenient for the corporations as very little effort was required. Unfortunately, despite the disadvantages to this system, it is often perpetuated solely because it is simple and is at this point a habit for most companies.
The internet has various features that are not available in Acrobat. A webpage can feature interactivity and multimedia elements such as video and audio, or even animations to further illustrate a point or to simply draw the reader’s attention. An HTML file can also be used on other mediums, such as PDAs or mobiles.
If the creator of the Acrobat file didn’t set it up to export as text, the file might be a series of graphics, with no text searching capabilities for the end user. This also means that search engines cannot search the document, so it won’t be shown to end users in their search results. And when Acrobat files DO show up in search results, the end user frequently will avoid clicking on that entry, knowing that loading an Acrobat file will slow down their search.
To find a small piece of information, the end user needs to wait for the entire Acrobat file to download, then they may need to find that file on their computer and manually launch it (depending on their settings). This might mean waiting for a hundred page document to download, then finding the appropriate heading in the table of contents, then advancing to that page number. On the internet, they would have simply clicked on the hyperlink for that page and would long since have found what they needed. (Acrobat DOES support hyperlinks, but many creators do not know how to correctly export with the hyperlinks intact, and many users of Reader do not expect hyperlinks in Acrobat so may not even realize that they can use them when they DO show up in a document.)
Remember that a primary objective of online posting of an annual report is to reach a new audience of potential investors. Waiting for a large file to download (that is likely to seem like a dry read once launched) is not the way to grab them and give them a good initial impression of your corporation. How many times have you been really enticed and captured by a PDF you found on the internet, and how many times have you been immediately interested in something that caught your attention on a web page? Unless you’re simply posting your annual reports out of obligation and have no interest in attracting new investors or providing your current ones with a satisfying experience, consider the distinct advantages of HTML.

