It is difficult to make a number of images into single pdf file. With manual method, usually you must copy into a word processing application and convert into a pdf.
But with ImageMagic, you can do it with just single line command.
First, you need to install ImageMagic;
$ sudo apt-get install imagemagick php5-imagick
Now, you can convert a number of images into single pdf like this;
$ convert images.pdf your-folder/*.jpg
If the result is blur, you can add options such as the following command;
$ convert -density 300 -quality 100 images.pdf your-folder/*.jpg
One more tips; If it is a series of images, you have to create an image sequence (rename) so the ImageMagic made in an order.