PDF is the newly supported format on iOS 8, in order to avoid the hassle of resizing icons for different device resolutions. Where I can get free / standard icon collection? For example,
Using a Advanced Google Image Search, you can search based on file type (SVG, not PDF) and also usage rights. If you want a common theme for all buttons, then you can use this tool to find single websites that host similarly designed images that meet your criteria.
Here is an example of the Resize Buttons with SVG. You'll have to filter by license for your use.
To find PDF files, you can't use the Image Search tool but you can use Google Advanced Search, and specify PDF as the file type.
To create or edit vector files (i.e. PDF or SVG), there is a very professional open source program called Inkscape (similar functionality to Adobe Illustrator). With this tool, you can also easily make your own buttons in vector format. You can also easily convert SVG to PDF, using tools described here.