There are a few ways you can import a PDF in Squid:
1. From the main screen
Tap the plus in the bottom right corner, then tap Import PDF. This will create a new note with the PDF you selected. If the PDF is one you've previously imported, you will have the option to create a new note or open an existing note containing the PDF.
2. While inside an existing note
You can insert a PDF anywhere inside an existing note (even a note that was created from a PDF). Any note can contain an infinite number of PDFs. To insert a PDF into an existing note, navigate to the page in the note where you want to insert the PDF, then long press the current page number, then choose Insert PDF.
3. From outside Squid
You can select a PDF while outside Squid and choose to open the PDF using Squid. This will create a new note with the PDF. If the PDF is one you've previously imported, you will have the option to create a new note or open an existing note containing the PDF.
For example, you could tap on a PDF attachment in your email app or tap on a PDF in Google Drive or Dropbox. Some apps may have an option titled Open with (or similar) when dealing with PDF files, which should show Squid as an option.
If you choose to make Squid your default app for handling PDFs, then opening a PDF from outside Squid will always import the PDF into Squid. Unfortunately, some devices (i.e. Samsung) will automatically make the app you choose the default app for opening PDFs. If you don't want this behavior, you can clear the default app in your device settings.