Can I take nasal decongestant with doxycycline?

Yes. There should be no interaction between any of the nasal decongestants: whether Pseudoephedrine or phenylpropanolamine, and doxycycline.
Decongestant use. Yes you can safely take a decongestant with an antibiotic such as doxycycline. There is no adverse interaction and no interference with the absorption of the antibiotic from taking the decongestant at the same time.