You can use PDFs in the following ways:
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Print out a physical copy and hand write on it.
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Import the PDF sheets to PDF annotation apps likes Goodnotes, Notability, Xodo, Noteshelf etc. You will then be able to add text/drawings/shapes/digital stickers on top.
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Add text directly onto the file using Adobe Reader (free) and preview. You can download the latest Adobe Acrobat Reader for FREE right here: https://get.adobe.com/reader/