Did you mean you want to convert some picture files to PDF? If so, you can do that with or without a 3rd party app.

i. Without a 3rd party app.
You can use the built-in Print function to convert a picture to a PDF. Here are the steps to do so on a Galaxy phone running Android Marshmallow:

1. Open the Gallery app.
2. View the picture you want to convert to PDF.
3. Tap Share.
4. Tap Print at the bottom of screen
5. Tap the PDF button in the upper right corner.
6. Select a folder to save the file.

ii. With a 3rd party app

With a 3rd party app such as Image to PDF Converter, you can select multiple pictures and convert them into a single PDF.

https://play.google.com/store/apps/details?id=bage.image2pdf

Thanks so much