Be sure to get a green book somewhere, which will list all of Portland's homeless resources for you, most notably the feeds. Lots of places have it, but the only one I can think of off the top of my head is Outside In (1132 SW 13th Ave, easy walking distance from downtown). The two feeds I regularly hit up in Portland are:
Sisters of the Road, which is less of a feed and more of a super cheap cafe ($1.00 for a plate of beans and rice or whatever the daily special is). Be careful here, though, lots of junkies and such hitting this up, last time I was in Portland, I'd heard that there were recently two overdose-related deaths there. If you have no money, you can work for meal tickets, so it's somewhat more dignified than hitting up a homebum feed.
And Blanchet House, on 340 NW Glisan ST, which feeds three times a day, 6:30-7:30 am for breakfast, 11:30-12:30 for lunch, and 5-6 for dinner. Long lines, pretty much all homebums, but the volunteers are very polite and will make you feel at home and help you keep your dignity.
Portland has some of the best homeless resources available out there (if you're not too proud to take advantage of them), just stay away from the Portland Rescue Mission.