You can import your PNG in Inkscape (File > Import), change the page dimensions if required via File > Document Properties, then click File > Save As, then select PDF from the file type options in the Save dialogue window.
This won't vectorize your PNG however, which may or may not be an issue, depending what you want to do with the PDF. If you need a vector object in your PDF, you can try running Trace Bitmap on your PNG (on the Path menu) first.
There are more detailed instructions on saving as a PDF here...
Beginners' Questions Converting a PNG to PDF
Inkscape.org
Can someone PLEASE tell me how I can convert a PNG to a PDF!! Thanks so much!
You can import your PNG in Inkscape (File > Import), change the page dimensions if required via File > Document Properties, then click File > Save As, then select PDF from the file type options in the Save dialogue window.
This won't vectorize your PNG however, which may or may not be an issue, depending what you want to do with the PDF. If you need a vector object in your PDF, you can try running Trace Bitmap on your PNG (on the Path menu) first.
There are more detailed instructions on saving as a PDF here...
https://inkscape-manuals.readthedocs.io/en/latest/export-pdf.html