While Adobe Acrobat is a great tool for you to create a PDF ebook, it is not free.
While you can convert HTML to PDF, and then combine all those PDF files into an e-book, it will be a little difficult.
So, how to create a PDF ebook free and easily? For WordPress users, you can check out the Anthologize plugin, which is available for you to transform your blog content into an ebook within the following 5 steps:
Step 1. Post your contents
Write all the contents you want to add into your ebook and post them on your WordPress blog, if you do not like to share them with the public, you can make their visibility as "Password protected" or "Private"; and if there are already hundreds of posts on your blog, you can even create a new one for your ebook, just like what I did.
Don't worry about the posts, since you can edit them any time later.
Step 2. Add new project
When the Anthologize plugin is installed and enabled, you can create the title, subtitle and/or author name of your ebook.
By the way, the subtitle and author name you created here won't be showed in your ebook indeed.
Step 3. Add new parts
A part can be taken as a chapter of your e-book, so you can create as many as you like.
Step 4. Add your items
Every post will be taken as an item under a part and will be listed in the table of contents in your ebook.
For all your WordPress blog posts, you can see and filter them by tags, time, categories, etc., you can drag any posts and drop them under any parts you like, and you can even enter another website's RSS feed URL to import its posts (up to 10).
By the way, you can edit the items later, and which won't reflect the original posts on your blog.
Step 5. Export your project
When the items are ready, you can export them to a PDF ebook (such as this), by choosing the copyright types, adding dedication/acknowledgements, selecting font size/face, breaking pages before parts and/or items, so on and so forth.
Besides PDF, you can also export your eBook as a RTF, HTML, ePub or XML (Anthologize TEI) file, but except the XML, all the other 3 format files are not readable during my test. And there will be more than one author name displayed in your PDF eBook, even though there is only one author.