It is not currently possible to write, draw or annotate on top of PDFs in Seesaw. We've had several requests for this feature, and hope to support it in the future. You can add a voice recording or text caption to PDFs.
In the meantime, if you want your students to be able to draw on top of an item, you will need to add it into Seesaw as an image. Many programs will allow you to convert PDFs into a JPG (on a Mac, you can use Preview.
On Macs or PCs, try http://smallpdf.com/pdf-to-jpg). You can also print a single copy of the worksheet, take a photo of it and upload that into Seesaw. Either of these options would allow your students to draw on top of the image.
On iOS or Android, you can take a screenshot of the page, and upload that screenshot into Seesaw to annotate on top.
Learn more about sending or pushing assignments to students here.