PDF Portfolios are usually sent as an email attachment to a job application (especially for applying to a to a remote job location) where they are judged before one is even chosen for an interview. For this reason it may include more extensive text for explanation in the form of labeling the images. These also can be inserted into online book/magazine sharing sites like Issuu (Links to an external site.) to be viewed by the public without having to build a a full web portfolio.
PDF portfolios can be created in a number of ways. If you already have a Traditional Portfolio you can copy these images with a scanner or camera, and depending on your device may save directly as a PDF, or import the images to a computer and make it in AdobeAcrobat, or you can build a Slideshow Portfolio with Google Slides (Links to an external site.), or PowerPoint Word (Links to an external site.) and save as a PDF. These can also can be made in LibreOffice (Links to an external site.) or GoogleDocs (Links to an external site.) like an illustrated paper or book before saving as a PDF.
If you want to build it in a format that not only makes for a highly controlled PDF "Book" but actually want to also be able to have copies printed out as an actual book ready for publication, you can use the Book Layout software InDesign (Links to an external site.):
InDesign Tutorial How to Design a Portfolio (Links to an external site.)
If you use Adobe PhotoShop to make, or crop, or correct color and are familiar with it, you can also build a PDF Portfolio directly in PhotoShop as well:
Export Multi Page PDF Portfolio in Adobe Photoshop (Links to an external site.)
Ask the professors in your discipline what would be good to include in your portfolio, and search online for similar input!