You can sign a PDF on your MacBook Air using Preview, which is included with macOS.
Preview can use your iPhone to create the signature through Continuity Markup, so you do not need to install a separate signing app for this method. (Apple Support)
Open the PDF in Preview, click Markup >> Sign, then choose Select Device and select your iPhone.
Sign on your iPhone with your finger and tap Done. You can then place and resize the signature in the PDF.
For this to work, Apple requires the Mac and iPhone to have Wi-Fi and Bluetooth turned on, be signed in to the same Apple Account, be within about 30 feet (10 meters) of each other, and meet Apple's Continuity Markup requirements. (Apple Support)
Your MacBook Air running macOS 26.6 and iPhone 14 Pro are within Apple's listed requirements for Continuity Markup. (Apple Support)
If your iPhone does not appear under Select Device, first check those Continuity requirements.
If you use a VPN, temporarily checking whether the problem persists without it may also be appropriate, because Apple notes that a VPN configuration can interfere with some Continuity features. (Apple Support)
Apple: Fill out and sign PDF forms in Preview on Mac