This is quick, easy, to the point and doesn't require any third-party script:
<embed src="http://example.com/the.pdf" width="500" height="375"
type="application/pdf">
UPDATE (2/3/2021)
Adobe now offers it's own PDF Embed API.
https://www.adobe.io/apis/documentcloud/dcsdk/pdf-embed.html
UPDATE (1/2018):
The Chrome browser on Android no longer supports PDF embeds. You can get around this by using the Google Drive PDF viewer
<embed src="https://drive.google.com/viewerng/
viewer?embedded=true&url=http://example.com/the.pdf" width="500" height="375">
Each YouTube video has four generated images. They are predictably formatted as follows:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/3.jpg
The first one in the list is a full size image and others are thumbnail images. The default thumbnail image (i.e., one of 1.jpg, 2.jpg, 3.jpg) is:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
For the high quality version of the thumbnail use a URL similar to this:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
There is also a medium quality version of the thumbnail, using a URL similar to the HQ:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
For the standard definition version of the thumbnail, use a URL similar to this:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
For the maximum resolution version of the thumbnail use a URL similar to this:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
All of the above URLs are available over HTTP too. Additionally, the slightly shorter hostname i3.ytimg.com works in place of img.youtube.com in the example URLs above.
Alternatively, you can use the YouTube Data API (v3) to get thumbnail images.
Best Answer
Here is the actual problem that occurred in your case.
When you embed a youtube video with the following code
<iframe src="https://www.youtube.com/v/VIDEO_ID"></iframe>a flash player is actually loaded. In this case your mobile doesn't support flash.To rectify this issue you can use an HTML5 player which has the following structure
<iframe src="https://www.youtube.com/embed/VIDEO_ID"></iframe>You can use this youtube code generator for generating customized player embed code.