  We present observations and initial analysis from an HST/STIS program to
obtain the first co-spatial, UV-optical spectra of ten Galactic planetary
nebulae (PNe). Our primary objective was to measure the critical emission lines
of carbon and nitrogen with unprecedented S/N and spatial resolution over
UV-optical range, with the ultimate goal of quantifying the production of these
elements in low- and intermediate-mass stars. Our sample was selected from PNe
with a near-solar metallicity, but spanning a broad range in N/O. This study,
the first of a series, concentrates on the observations and emission-line
measurements obtained by integrating along the entire spatial extent of the
slit. We derived ionic and total elemental abundances for the seven PNe with
the strongest UV line detections (IC~2165, IC~3568, NGC~2440, NGC~3242,
NGC~5315, NGC~5882, and NGC~7662). We compare these new results with other
recent studies of the nebulae, and discuss the relative merits of deriving the
total elemental abundances of C, N, and O using ionization correction factors
(ICFs) versus summed abundances. For the seven PNe with the best UV line
detections, we conclude that summed abundances from direct diagnostics of ions
with measurable UV lines gives the most accurate values for the total elemental
abundances of C and N. In some cases where significant discrepancies exist
between our abundances and those from other studies, we show that the
differences can often be attributed to their use of fluxes that are not
co-spatial. Finally, we examined C/O and N/O versus O/H and He/H in
well-observed Galactic, LMC, and SMC PNe, and found that highly accurate
abundances are essential for properly inferring elemental yields from their
progenitor stars.
